LT3991/LT3991-3.

3/LT3991-5
55V, 1.2A Step-Down
Regulator with 2.8µA
Quiescent Current
Features Description
n Ultralow Quiescent Current: The LT®3991 is an adjustable frequency monolithic buck
2.8µA IQ Regulating 12VIN to 3.3VOUT switching regulator that accepts a wide input voltage range
n Fixed Output Voltages: 3.3V, 5V up to 55V. Low quiescent current design consumes only
2.1µA IQ Regulating 12VIN 2.8µA of supply current while regulating with no load. Low
n Low Ripple Burst Mode® Operation: ripple Burst Mode operation maintains high efficiency at
Output Ripple < 15mVP-P low output currents while keeping the output ripple below
n Wide Input Voltage Range: 4.3V to 55V 15mV in a typical application. An internally compensated
n 1.2A Maximum Output Current current mode topology is used for fast transient response
n Adjustable Switching Frequency: 200kHz to 2MHz and good loop stability. A high efficiency 0.44Ω switch
n Synchronizable Between 250kHz to 2MHz is included on the die along with a boost Schottky diode
n Fast Transient Response and the necessary oscillator, control and logic circuitry.
n Accurate 1V Enable Pin Threshold An accurate 1V threshold enable pin can be used to shut
n Low Shutdown Current: IQ = 700nA down the LT3991, reducing the input supply current to
n Power Good Flag 700nA. A capacitor on the SS pin provides a controlled
n Soft-Start CapabilityV inrush current (soft-start). A power good flag signals
n Internal Compensation when VOUT reaches 91% of the programmed output volt-
n Saturating Switch Design: 0.44Ω On-Resistance age. The LT3991 is available in small 10-pin MSOP and
n Output Voltage: 1.19V to 30V 3mm × 3mm DFN packages with exposed pads for low
n Small Thermally Enhanced 10-Pin MSOP Package thermal resistance.
and (3mm × 3mm) DFN Packages L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective

Applications owners.

n Automotive Battery Regulation

n Power for Portable Products
n Industrial Supplies

Typical Application No Load Supply Current

3.0
3.3V Step Down Converter OUTPUT IN REGULATION

VIN
4.3V TO 55V 2.5
INPUT CURRENT (µA)

VIN
OFF ON EN./UVLO BOOST
0.47µF LT3991-5
PG 12µH 2.0
SW
SS LT3991-3.3 LT3991-3.3
4.7µF
RT 1.5
BD VOUT
118k VOUT 3.3V
SYNC GND 1.2A
47µF 1.0
5 10 15 20 25 30 35 40 45 50 55
f = 400kHz 3991 TA01a
INPUT VOLTAGE (V)
3991 G06

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LT3991/LT3991-3.3/LT3991-5
Absolute Maximum Ratings
(Note 1)
VIN, EN Voltage..........................................................55V Operating Junction Temperature Range (Note 2)
BOOST Pin Voltage....................................................75V LT3991E.............................................. –40°C to 125°C
BOOST Pin Above SW Pin..........................................30V LT3991I............................................... –40°C to 125°C
FB, VOUT, RT, SYNC, SS Voltage..................................6V Storage Temperature Range............... –65°C to 150°C
PG, BD Voltage..........................................................30V Lead Temperature (Soldering, 10 sec)
Boost Diode Current.....................................................1A (MSE Only)........................................................ 300°C

Pin Configuration
LT3991 LT3991 LT3991-3.3, LT3991-5

TOP VIEW

TOP VIEW TOP VIEW

BD 1 10 SYNC
BD 1 10 SYNC BD 1 10 SYNC
BOOST 2 9 PG BOOST 2 9 PG BOOST 2 9 PG
11 11 11
SW 3 8 RT SW 3 8 RT SW 3 8 RT
GND GND GND
VIN 4 7 SS VIN 4 7 SS
VIN 4 7 SS
EN 5 6 FB EN 5 6 VOUT
EN 5 6 FB
MSE PACKAGE MSE PACKAGE
10-LEAD PLASTIC MSOP 10-LEAD PLASTIC MSOP
DD PACKAGE
θJA = 45°C, θJC = 10°C/W θJA = 45°C, θJC = 10°C/W
10-LEAD (3mm × 3mm) PLASTIC DFN
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
θJA = 45°C, θJC = 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB

Order Information
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3991EDD#PBF LT3991EDD#TRPBF LFJR 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C
LT3991IDD#PBF LT3991IDD#TRPBF LFJR 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C
LT3991EMSE#PBF LT3991EMSE#TRPBF LTFJS 10-Lead Plastic MSOP –40°C to 125°C
LT3991IMSE#PBF LT3991IMSE#TRPBF LTFJS 10-Lead Plastic MSOP –40°C to 125°C
LT3991EMSE-3.3#PBF LT3991EMSE-3.3#TRPBF LTFRS 10-Lead Plastic MSOP –40°C to 125°C
LT3991IMSE-3.3#PBF LT3991IMSE-3.3#TRPBF LTFRS 10-Lead Plastic MSOP –40°C to 125°C
LT3991EMSE-5#PBF LT3991EMSE-5#TRPBF LTFRV 10-Lead Plastic MSOP –40°C to 125°C
LT3991IMSE-5#PBF LT3991IMSE-5#TRPBF LTFRV 10-Lead Plastic MSOP –40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/

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 LT3991/LT3991-3.3/LT3991-5
Electrical Characteristics The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VEN = 12V, VBD = 3.3V unless otherwise noted. (Note 2)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage (Note 4) l 4 4.3 V
Quiescent Current from VIN VEN Low 0.7 1.2 μA
VEN High, VSYNC Low 1.7 2.7 μA
VEN High, VSYNC Low l 4.5 μA
LT3991 FB Pin Current VFB = 1.19V l 0.1 12 nA
Internal Feedback Resistor Divider 10 MΩ
Feedback Voltage 1.175 1.19 1.205 V
l 1.165 1.19 1.215 V
LT3991-3.3 Output Voltage 3.25 3.3 3.35 V
l 3.224 3.3 3.376 V
LT3991-5 Output Voltage 4.93 5 5.07 V
l 4.89 5 5.11 V
FB Voltage Line Regulation 4.3V < VIN < 40V (Note 4) 0.0002 0.01 %/V
Switching Frequency RT = 11k 1.6 2 2.4 MHz
RT = 35.7k 0.8 1 1.2 MHz
RT = 255k 160 200 240 kHz
Minimum Switch On Time 110 ns
Minimum Switch Off Time 150 200 ns
Switch Current Limit 1.7 2.3 2.9 A
Switch VCESAT ISW = 1A 440 mV
Switch Leakage Current 0.02 1 μA
Boost Schottky Forward Voltage ISH = 100mA 800 mV
Boost Schottky Reverse Leakage VREVERSE = 12V 0.02 1 μA
Minimum Boost Voltage (Note 3) VIN = 5V l 1.4 1.8 V
BOOST Pin Current ISW = 1A, VBOOST = 15V 25 33 mA
EN Voltage Threshold EN Rising l 0.95 1.01 1.07 V
EN Voltage Hysteresis 30 mV
EN Pin Current 0.2 20 nA
LT3991 PG Threshold Offset from VFB VFB Rising 60 100 140 mV
LT3991 PG Hysteresis 20 mV
LT3991-X PG Threshold Offset from VOUT VOUT Rising 5.5 9 12.5 %
LT3991X PG Hysteresis 1.3 %
PG Leakage VPG = 3V 0.02 1 µA
PG Sink Current VPG = 0.4V l 300 570 μA
SYNC Threshold 0.6 0.8 1.0 V
SYNC Pin Current 0.1 nA
SS Source Current VSS = 1V 0.6 1 1.6 μA

Note 1: Stresses beyond those listed under Absolute Maximum Ratings The LT3991I is guaranteed over the full –40°C to 125°C operating junction
may cause permanent damage to the device. Exposure to any Absolute temperature range. High junction temperatures degrade operating
Maximum Rating condition for extended periods may affect device lifetimes. Operating lifetime is derated at junction temperatures greater
reliability and lifetime. than 125°C.
Note 2: The LT3991E is guaranteed to meet performance specifications Note 3: This is the minimum voltage across the boost capacitor needed to
from 0°C to 125°C junction temperature. Specifications over the –40°C guarantee full saturation of the switch.
to 125°C operating junction temperature range are assured by design, Note 4: Minimum input voltage depends on application circuit.
characterization, and correlation with statistical process controls.
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 LT3991/LT3991-3.3/LT3991-5
Typical Performance Characteristics TA = 25°C, unless otherwise noted.

Efficiency, VOUT = 5V Efficiency, VOUT = 3.3V Efficiency, VOUT = 5V

100 90 100
VIN = 12V VOUT = 5V
VIN = 12V 90 R1 = 1M
90 80 R2 = 309k
80
VIN = 12V
80 70 VIN = 24V 70

EFFICIENCY (%)

EFFICIENCY (%)
EFFICIENCY (%)

VIN = 24V VIN = 36V

60
70 60
VIN = 36V VIN = 48V 50 VIN = 24V
60 VIN = 48V 50
40
VIN = 36V
50 40 30
VIN = 48V
20
40 VOUT = 5V 30
R1 = 1M 10
R2 = 309k
30 20 0
0 0.2 0.4 0.6 0.8 1 1.2 0 0.2 0.4 0.6 0.8 1 1.2 0.01 0.1 1 10 100 1000
LOAD CURRENT (A) LOAD CURRENT (A) LOAD CURRENT (mA)
3991 G01 3991 G02 3991 G03

Efficiency, VOUT = 3.3V No Load Supply Current No Load Supply Current

90 100 3.0
DIODES, INC. OUTPUT IN REGULATION
80 DFLS2100

70 VIN = 12V
2.5
VIN = 24V

INPUT CURRENT (µA)

INPUT CURRENT (µA)

60
EFFICIENCY (%)

VIN = 36V
50 LT3991-5
VIN = 48V 10 2.0
40
LT3991-3.3
30

20 1.5

10

0 1 1.0
0.01 0.1 1 10 100 1000 –55 –25 5 35 65 95 125 155 5 10 15 20 25 30 35 40 45 50 55
LOAD CURRENT (mA) TEMPERATURE (°C) INPUT VOLTAGE (V)
3991 G04 3991 G05
3991 G06

LT3991 Feedback Voltage LT3991-3.3 Output Voltage LT3991-5 Output Voltage

1.205 3.345 5.06

1.200 3.330 5.04

FEEDBACK VOLTAGE (V)

OUTPUT VOLTAGE (V)

OUTPUT VOLTAGE (V)

1.195 3.315 5.02

1.190 3.300 5.00

1.185 3.285 4.98

1.180 3.270 4.96

1.175 3.255 4.94

–55 –25 5 35 65 95 125 155 –50 –25 5 35 65 95 125 155 –50 –25 5 35 65 95 125 155
TEMPERATURE (°C) TEMPERATURE (°C) TEMPERATURE (°C)
3991 G07
3991 G28 3991 G29

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 LT3991/LT3991-3.3/LT3991-5
Typical Performance Characteristics TA = 25°C, unless otherwise noted.

Maximum Load Current Maximum Load Current Load Regulation

3.0 2.5 0.30
VOUT = 3.3V VOUT = 5V REFERENCED FROM VOUT AT 0.5A LOAD
0.25
2.5 0.20
2.0
0.15

LOAD REGULATION (%)

TYPICAL

LOAD CURRENT (A)

LOAD CURRENT (A)

2.0 TYPICAL 0.10

1.5 0.05
MINIMUM
1.5 MINIMUM 0
1.0 –0.05
1.0 –0.10
–0.15
0.5
0.5 –0.20
–0.25
0 0 –0.30
5 10 15 20 25 30 35 40 45 50 55 5 10 15 20 25 30 35 40 45 50 55 0 200 400 600 800 1000 1200
INPUT VOLTAGE (V) INPUT VOLTAGE (V) LOAD CURRENT (mA)
3991 G08 3991 G09 3991 G10

Switching Frequency Switch Current Limit Switch Current Limit

1000 3.0 2.5

950 2.4
2.5
2.3

SWITCH CURRENT LIMIT (A)

SWITCH CURRENT LIMIT (A)

900
2.2
2.0
FREQUENCY (kHz)

850 2.1
800 1.5 2.0

750 1.9
1.0
1.8
700
1.7
0.5
650 1.6
DUTY CYCLE = 30%
600 0 1.5
–55 –25 5 35 65 95 125 155 0 20 40 60 80 100 –55 –25 5 35 65 95 125 155
TEMPERATURE (°C) DUTY CYCLE (%) TEMPERATURE (°C)
3991 G11 3991 G12 3991 G13

Switch VCESAT Boost Pin Current LT3991 Frequency Foldback

700 45 900

40 800
600
SWITCHING FREQUENCY (kHz)

35 700
BOOST PIN CURRENT (mA)

500
30 600
VCESAT (mV)

400 25 500

300 20 400

15 300
200
10 200
100
5 100

0 0 0
0 250 500 750 1000 1250 1500 0 250 500 750 1000 1250 1500 0 0.2 0.4 0.6 0.8 1 1.2
SWITCH CURRENT (mA) SWITCH CURRENT (mA) FB PIN VOLTAGE (V)
3991 G14 3991 G15 3991 G16

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LT3991/LT3991-3.3/LT3991-5
Typical Performance Characteristics TA = 25°C, unless otherwise noted.

Minimum Switch On-Time/

LT3991-X Frequency Foldback Switch Off-Time Soft-Start
900 400 2.5

800 350
tOFF(MIN) 1A LOAD 2.0
SWITCHING FREQUENCY (kHz)

700

SWITCH CURRENT LIMIT (A)

SWITCH ON/OFF TIME (ns)
300
600
250
1.5
500 tOFF(MIN) 0.5A LOAD
200
400
150 1.0
300
100 tON(MIN)
200 0.5
100 50

0 0 0
0 20 40 60 80 100 –55 –25 5 35 65 95 125 155 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
VOUT (% OF REGULATION VOLTAGE) TEMPERATURE (°C) SS PIN VOLTAGE (V)
3991 G17 3991 G18
3991 G30

Minimum Input Voltage Minimum Input Voltage EN Threshold

5.0 6.4 1.05
VOUT = 3.3V VOUT = 5V
4.8 1.04
6.2
4.6 1.03

THRESHOLD VOLTAGE (V)

4.4 6.0 1.02
INPUT VOLTAGE (V)

INPUT VOLTAGE (V)

TO START
RISING THRESHOLD
4.2 TO START 1.01
5.8
4.0 1.00
5.6
3.8 0.99
TO RUN FALLING THRESHOLD
3.6 5.4 0.98
3.4 0.97
5.2 TO RUN
3.2 0.96
3.0 5.0 0.95
0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 –55 –25 5 35 65 95 125 155
LOAD CURRENT (mA) LOAD CURRENT (mA) TEMPERATURE (°C)
3991 G19 3991 G20 3971 G21

Transient Load Response,

Load Current Stepped from 25mA
Boost Diode Forward Voltage Power Good Threshold (Burst Mode Operation) to 525mA
1.6 95

1.4 94
93
VOUT
THRESHOLD VOLTAGE (%)

1.2
92 100mV/DIV
BOOST DIODE VF (V)

1.0 91
0.8 90

0.6 89
88
0.4
87
IL
0.2 86 500mA/DIV
0 85
0 250 500 750 1000 1250 1500 –55 –25 5 35 65 95 125 155 10µs/DIV 3991 G24

BOOST DIODE CURRENT (mA) TEMPERATURE (°C) VIN = 48V, VOUT = 3.3V
3991 G22 3991 G23
COUT = 47µF

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 LT3991/LT3991-3.3/LT3991-5
Typical Performance Characteristics TA = 25°C, unless otherwise noted.

Transient Load Response,

Load Current Stepped from Switching Waveforms; Switching Waveforms; Full
0.5A to 1A Burst Mode Operation Frequency Continuous Operation

VOUT
100mV/ VSW
VSW
DIV 5V/DIV
5V/DIV

IL IL
500mA/DIV 500mA/DIV
IL
500mA/
DIV VOUT VOUT
20mV/DIV 20mV/DIV

3971 G26
10µs/DIV 3991 G25
5µs/DIV 1µs/DIV 3971 G27

VIN = 48V, VOUT = 3.3V VIN = 48V, VOUT = 3.3V VIN = 48V, VOUT = 3.3V
COUT = 47µF ILOAD = 20mA ILOAD = 1A
COUT = 47µF COUT = 47µF

Pin Functions
BD (Pin 1): This pin connects to the anode of the boost FB (Pin 6, LT3991 Only): The LT3991 regulates the FB pin
diode. The BD pin is normally connected to the output. to 1.19V. Connect the feedback resistor divider tap to this
pin. Also, connect a phase lead capacitor between FB and
BOOST (Pin 2): This pin is used to provide a drive volt-
age, higher than the input voltage, to the internal bipolar VOUT. Typically this capacitor is 10pF.
NPN power switch. VOUT (Pin 6, LT3991-3.3/LT3991-5 Only): The LT3991-
3.3 and LT3991-5 regulate the VOUT pin to 3.3V and 5V
SW (Pin 3): The SW pin is the output of an internal power
respectively. This pin connects to the internal 10MΩ
switch. Connect this pin to the inductor, catch diode, and
feedback divider that programs the fixed output voltage.
boost capacitor.
SS (Pin 7): A capacitor and a series resistor are tied between
VIN (Pin 4): The VIN pin supplies current to the LT3991’s
internal circuitry and to the internal power switch. This SS and ground to slowly ramp up the peak current limit
pin must be locally bypassed. of the LT3991 on start-up. The soft-start capacitor is only
actively discharged when EN is low. The SS pin is released
EN (Pin 5): The part is in shutdown when this pin is low when the EN pin goes high. Float this pin to disable soft-
and active when this pin is high. The hysteretic threshold start. The soft-start resistor has a typical value of 100k.
voltage is 1.005V going up and 0.975V going down. The EN
threshold is only accurate when VIN is above 4.3V. If VIN is RT (Pin 8): A resistor is tied between RT and ground to
lower than 4.3V, ground EN to place the part in shutdown. set the switching frequency.
Tie to VIN if shutdown feature is not used.

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LT3991/LT3991-3.3/LT3991-5
Pin Functions
PG (Pin 9): The PG pin is the open-drain output of an at low output loads. Tie to a clock source for synchroni-
internal comparator. PGOOD remains low until the FB pin zation, which will include pulse-skipping at low output
is within 9% of the final regulation voltage. PGOOD is loads. When in pulse-skipping mode, quiescent current
valid when the LT3991 is enabled and VIN is above 4.3V. increases to 1.5mA.
SYNC (Pin 10): This is the external clock synchronization GND (Exposed Pad Pin 11): Ground. The exposed pad
input. Ground this pin for low ripple Burst Mode operation must be soldered to PCB.

Block Diagram

VIN
C1
VIN –
INTERNAL 1.19V REF
+ BD
1V + SWITCH
– SHDN Σ SLOPE COMP LATCH
BOOST
EN
R
OSCILLATOR Q C3
RT 200kHz TO 2MHz S
RT L1
VOUT
SW
Burst Mode
SYNC DETECT D1 C2

PG ERROR AMP
VC CLAMP
+ 1.09V + VC
1µA
– –
SS
C4
SHDN
C5 LT3991-3.3
LT3991-5
ONLY
R2 R1

GND FB LT3991 VOUT

ONLY

R2 R1
3991 BD

C5

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 LT3991/LT3991-3.3/LT3991-5
Operation
The LT3991 is a constant frequency, current mode step- voltage at the BOOST pin that is higher than the input
down regulator. An oscillator, with frequency set by RT, supply. This allows the driver to fully saturate the internal
sets an RS flip-flop, turning on the internal power switch. bipolar NPN power switch for efficient operation.
An amplifier and comparator monitor the current flowing To further optimize efficiency, the LT3991 automatically
between the VIN and SW pins, turning the switch off when switches to Burst Mode operation in light load situations.
this current reaches a level determined by the voltage at
Between bursts, all circuitry associated with controlling
VC (see Block Diagram). An error amplifier measures the
the output switch is shut down, reducing the input supply
output voltage through an external resistor divider tied to
current to 1.7μA. In a typical application, 2.8μA will be con-
the FB pin and servos the VC node. If the error amplifier’s sumed from the supply when regulating with no load.
output increases, more current is delivered to the output;
if it decreases, less current is delivered. An active clamp The oscillator reduces the LT3991’s operating frequency
on the VC node provides current limit. The VC node is when the voltage at the FB pin is low. This frequency
also clamped by the voltage on the SS pin; soft-start is foldback helps to control the output current during start-
implemented by generating a voltage ramp at the SS pin up and overload.
using an external capacitor and resistor. The LT3991 contains a power good comparator which
If the EN pin is low, the LT3991 is shut down and draws trips when the FB pin is at 91% of its regulated value. The
700nA from the input. When the EN pin exceeds 1.01V, PG output is an open-drain transistor that is off when the
the switching regulator will become active. output is in regulation, allowing an external resistor to pull
the PG pin high. Power good is valid when the LT3991 is
The switch driver operates from either VIN or from the
enabled and VIN is above 4.3V.
BOOST pin. An external capacitor is used to generate a

Applications Information
1000
Achieving Ultralow Quiescent Current VIN = 12V
VOUT = 3.3V
To enhance efficiency at light loads, the LT3991 operates 800
SWITCHING FREQUENCY (kHz)

in low ripple Burst Mode, which keeps the output capacitor

charged to the desired output voltage while minimizing 600

the input quiescent current. In Burst Mode operation the

LT3991 delivers single pulses of current to the output ca- 400

pacitor followed by sleep periods where the output power

is supplied by the output capacitor. When in sleep mode 200

the LT3991 consumes 1.7μA, but when it turns on all the

0
circuitry to deliver a current pulse, the LT3991 consumes 0 20 40 60 80 100 120
1.5mA of input current in addition to the switch current. LOAD CURRENT (mA)
3991 F01

Therefore, the total quiescent current will be greater than

1.7μA when regulating. Figure 1. Switching Frequency in Burst Mode Operation

As the output load decreases, the frequency of single cur- quiescent current performance at light loads, the current
rent pulses decreases (see Figure 1) and the percentage in the feedback resistor divider and the reverse current
of time the LT3991 is in sleep mode increases, resulting in the catch diode must be minimized, as these appear
in much higher light load efficiency. By maximizing the to the output as load currents. Use the largest possible
time between pulses, the converter quiescent current feedback resistors and a low leakage Schottky catch diode
gets closer to the 1.7μA ideal. Therefore, to optimize the in applications utilizing the ultralow quiescent current
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LT3991/LT3991-3.3/LT3991-5
Applications Information
performance of the LT3991. The feedback resistors should quiescent current will significantly increase to 1.5mA in
preferably be on the order of MΩ and the Schottky catch light load situations when synchronized with an external
diode should have less than 1µA of typical reverse leak- clock. Holding the SYNC pin high yields no advantages in
age at room temperature. These two considerations are terms of output ripple or minimum load to full frequency,
reiterated in the FB Resistor Network and Catch Diode so is not recommended.
Selection sections.
FB Resistor Network
It is important to note that another way to decrease the
pulse frequency is to increase the magnitude of each The output voltage is programmed with a resistor divider
single current pulse. However, this increases the output between the output and the FB pin. Choose the resistor
voltage ripple because each cycle delivers more power to values according to:
the output capacitor. The magnitude of the current pulses ⎛V ⎞
was selected to ensure less than 15mV of output ripple in R1= R2 ⎜ OUT − 1⎟
a typical application. See Figure 2. ⎝ 1.19V ⎠
Reference designators refer to the Block Diagram. 1%
VSW
5V/DIV resistors are recommended to maintain output voltage
accuracy.
IL
The total resistance of the FB resistor divider should be
500mA/DIV selected to be as large as possible to enhance low current
performance. The resistor divider generates a small load
VOUT
20mV/DIV
on the output, which should be minimized to optimize the
low supply current at light loads.
3991 F02
5µs/DIV
VIN = 48V When using large FB resistors, a 10pF phase lead capacitor
VOUT = 3.3V
ILOAD = 20mA should be connected from VOUT to FB.

Figure 2. Burst Mode Operation The LT3991-3.3 and LT3991-5 control an internal 10M FB
resistor divider as well as an internal lead capacitor.
While in Burst Mode operation, the burst frequency and the
charge delivered with each pulse will not change with output Setting the Switching Frequency
capacitance. Therefore, the output voltage ripple will be
The LT3991 uses a constant frequency PWM architecture
inversely proportional to the output capacitance. In a typical
that can be programmed to switch from 200kHz to 2MHz
application with a 47μF output capacitor, the output ripple
by using a resistor tied from the RT pin to ground. A table
is about 8mV, and with a 100μF output capacitor the output
showing the necessary RT value for a desired switching
ripple is about 4mV. The output voltage ripple can continue
frequency is in Table 1.
to be decreased by increasing the output capacitance.
Table 1. Switching Frequency vs RT Value
At higher output loads (above 86mA for the front page
SWITCHING FREQUENCY (MHz) RT VALUE (kΩ)
application) the LT3991 will be running at the frequency
0.2 255
programmed by the RT resistor, and will be operating in 0.4 118
standard PWM mode. The transition between PWM and low 0.6 71.5
0.8 49.9
ripple Burst Mode operation will exhibit slight frequency 1.0 35.7
jitter, but will not disturb the output voltage. 1.2 28.0
1.4 22.1
To ensure proper Burst Mode operation, the SYNC pin 1.6 17.4
must be grounded. When synchronized with an exter- 1.8 14.0
2.0 11.0
nal clock, the LT3991 will pulse skip at light loads. The
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 LT3991/LT3991-3.3/LT3991-5
Applications Information
Operating Frequency Tradeoffs Input Voltage Range
Selection of the operating frequency is a tradeoff between The minimum input voltage is determined by either the
efficiency, component size, minimum dropout voltage, and LT3991’s minimum operating voltage of 4.3V or by its
maximum input voltage. The advantage of high frequency maximum duty cycle (see equation in Operating Frequency
operation is that smaller inductor and capacitor values may Tradeoffs section). The minimum input voltage due to
be used. The disadvantages are lower efficiency, lower duty cycle is:
maximum input voltage, and higher dropout voltage. The VOUT + VD
highest acceptable switching frequency (fSW(MAX)) for a VIN(MIN) = − VD + VSW
1− fSW tOFF(MIN)
given application can be calculated as follows:
VOUT + VD where VIN(MIN) is the minimum input voltage, VOUT is
fSW(MAX) = the output voltage, VD is the catch diode drop (~0.5V),
tON(MIN)(VIN − VSW + VD )
VSW is the internal switch drop (~0.5V at max load), fSW
is the switching frequency (set by RT), and tOFF(MIN) is
where VIN is the typical input voltage, VOUT is the output
the minimum switch off-time. Note that higher switch-
voltage, VD is the catch diode drop (~0.5V), and VSW is
ing frequency will increase the minimum input voltage.
the internal switch drop (~0.5V at max load). This equation If a lower dropout voltage is desired, a lower switching
shows that slower switching frequency is necessary to frequency should be used.
safely accommodate high VIN/VOUT ratio. Also, as shown
in the Input Voltage Range section, lower frequency allows The maximum input voltage for LT3991 applications
a lower dropout voltage. The input voltage range depends depends on switching frequency, the Absolute Maximum
on the switching frequency because the LT3991 switch has Ratings of the VIN and BOOST pins, and the operating
finite minimum on and off times. The minimum switch on mode. For a given application where the switching fre-
and off times are strong functions of temperature. Use quency and the output voltage are already selected, the
the typical minimum on and off curves to design for an maximum input voltage (VIN(OP-MAX)) that guarantees
application’s maximum temperature, while adding about optimum output voltage ripple for that application can be
30% for part-to-part variation. The minimum and maximum found by applying the following equation:
duty cycles that can be achieved taking minimum on and VOUT + VD
off times into account are: VIN(OP-MAX) = – VD + VSW
fSW • tON(MIN)
DCMIN = fSW tON(MIN)
where tON(MIN) is the minimum switch on-time. Note that
DCMAX = 1− fSW tOFF(MIN) a higher switching frequency will decrease the maximum
operating input voltage. Conversely, a lower switching
where fSW is the switching frequency, the tON(MIN) is the frequency will be necessary to achieve normal operation
minimum switch on-time, and the tOFF(MIN) is the minimum at higher input voltages.
switch off-time. These equations show that duty cycle
The circuit will tolerate inputs above the maximum op-
range increases when switching frequency is decreased.
erating input voltage and up to the Absolute Maximum
See the Electrical Characteristics section for tON(MIN) and
Ratings of the VIN and BOOST pins, regardless of chosen
tOFF(MIN) values.
switching frequency. However, during such transients
A good choice of switching frequency should allow ad- where VIN is higher than VIN(OP-MAX), the LT3991 will enter
equate input voltage range (see Input Voltage Range sec- pulse-skipping operation where some switching pulses are
tion) and keep the inductor and capacitor values small. skipped to maintain output regulation. The output voltage
ripple and inductor current ripple will be higher than in
typical operation. Do not overload when VIN is greater
than VIN(OP-MAX).
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LT3991/LT3991-3.3/LT3991-5
Applications Information
Inductor Selection and Maximum Output Current When the switch is off, the potential across the inductor
A good first choice for the inductor value is: is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor:
VOUT + VD
L= (1−DC) • (VOUT + VD )
fSW ΔIL =
L • fSW
where fSW is the switching frequency in MHz, VOUT is the
where fSW is the switching frequency of the LT3991, DC is
output voltage, VD is the catch diode drop (~0.5V) and L
the duty cycle and L is the value of the inductor. Therefore,
is the inductor value in μH.
the maximum output current that the LT3991 will deliver
The inductor’s RMS current rating must be greater than the depends on the switch current limit, the inductor value,
maximum load current and its saturation current should be and the input and output voltages. The inductor value may
about 30% higher. For robust operation in fault conditions have to be increased if the inductor ripple current does
(start-up or short-circuit) and high input voltage (>30V), not allow sufficient maximum output current (IOUT(MAX))
the saturation current should be above 2.8A. To keep the given the switching frequency, and maximum input voltage
efficiency high, the series resistance (DCR) should be less used in the desired application.
than 0.1Ω, and the core material should be intended for
The optimum inductor for a given application may differ
high frequency applications. Table 2 lists several vendors
from the one indicated by this simple design guide. A larger
and suitable types.
value inductor provides a higher maximum load current and
The inductor value must be sufficient to supply the desired reduces the output voltage ripple. If your load is lower than
maximum output current (IOUT(MAX)), which is a function the maximum load current, than you can relax the value of
of the switch current limit (ILIM) and the ripple current. the inductor and operate with higher ripple current. This
ΔIL allows you to use a physically smaller inductor, or one with
IOUT(MAX) = ILIM – a lower DCR resulting in higher efficiency. Be aware that if
2 the inductance differs from the simple rule above, then the
The LT3991 limits its peak switch current in order to protect maximum load current will depend on the input voltage. In
itself and the system from overload faults. The LT3991’s addition, low inductance may result in discontinuous mode
switch current limit (ILIM) is at least 2.33A at low duty operation, which further reduces maximum load current.
cycles and decreases linearly to 1.8A at DC = 0.8. For details of maximum output current and discontinuous
Table 2. Inductor Vendors
operation, see Linear Technology’s Application Note 44.
Finally, for duty cycles greater than 50% (VOUT/VIN>0.5),
VENDOR URL PART SERIES TYPE
a minimum inductance is required to avoid sub-harmonic
Murata www.murata.com LQH55D Open
oscillations. See Application Note 19.
TDK www.componenttdk.com SLF7045 Shielded
SLF10145 Shielded One approach to choosing the inductor is to start with
Toko www.toko.com D62CB Shielded the simple rule given above, look at the available induc-
D63CB Shielded
D73C Shielded
tors, and choose one to meet cost or space goals. Then
D75F Open use the equations above to check that the LT3991 will be
Coilcraft www.coilcraft.com MSS7341 Shielded able to deliver the required output current. Note again
MSS1038 Shielded that these equations assume that the inductor current is
Sumida www.sumida.com CR54 Open continuous. Discontinuous operation occurs when IOUT
CDRH74 Shielded
CDRH6D38 Shielded is less than ΔIL/2.
CR75 Open

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 LT3991/LT3991-3.3/LT3991-5
Applications Information
Input Capacitor equivalent series resistance (ESR) and provide the best
Bypass the input of the LT3991 circuit with a ceramic ripple performance. A good starting value is:
capacitor of X7R or X5R type. Y5V types have poor 100
COUT =
performance over temperature and applied voltage, and VOUT fSW
should not be used. A 4.7μF to 10μF ceramic capacitor
is adequate to bypass the LT3991 and will easily handle where fSW is in MHz, and COUT is the recommended output
the ripple current. Note that larger input capacitance is capacitance in μF. Use X5R or X7R types. This choice will
required when a lower switching frequency is used (due provide low output ripple and good transient response.
to longer on-times). If the input power source has high Transient performance can be improved with a higher
impedance, or there is significant inductance due to value capacitor. Increasing the output capacitance will
long wires or cables, additional bulk capacitance may be also decrease the output voltage ripple. A lower value of
necessary. This can be provided with a low performance output capacitor can be used to save space and cost but
electrolytic capacitor. transient performance will suffer.

Step-down regulators draw current from the input sup- When choosing a capacitor, look carefully through the
ply in pulses with very fast rise and fall times. The input data sheet to find out what the actual capacitance is under
capacitor is required to reduce the resulting voltage operating conditions (applied voltage and temperature). A
ripple at the LT3991 and to force this very high frequency physically larger capacitor or one with a higher voltage rating
switching current into a tight local loop, minimizing EMI. may be required. Table 3 lists several capacitor vendors.
A 4.7μF capacitor is capable of this task, but only if it is Table 3. Recommended Ceramic Capacitor Vendors
placed close to the LT3991 (see the PCB Layout section). MANUFACTURER WEBSITE
A second precaution regarding the ceramic input capacitor AVX www.avxcorp.com
concerns the maximum input voltage rating of the LT3991. Murata www.murata.com
A ceramic input capacitor combined with trace or cable Taiyo Yuden www.t-yuden.com
inductance forms a high quality (under damped) tank cir- Vishay Siliconix www.vishay.com
cuit. If the LT3991 circuit is plugged into a live supply, the TDK www.tdk.com
input voltage can ring to twice its nominal value, possibly
exceeding the LT3991’s voltage rating. This situation is
easily avoided (see the Hot Plugging Safely section). Catch Diode Selection
The catch diode (D1 from Block Diagram) conducts cur-
Output Capacitor and Output Ripple rent only during switch off time. Average forward current
The output capacitor has two essential functions. Along in normal operation can be calculated from:
with the inductor, it filters the square wave generated by the VIN – VOUT
LT3991 to produce the DC output. In this role it determines ID(AVG) = IOUT
VIN
the output ripple, so low impedance (at the switching
frequency) is important. The second function is to store where IOUT is the output load current. The only reason to
energy in order to satisfy transient loads and stabilize the consider a diode with a larger current rating than necessary
LT3991’s control loop. Ceramic capacitors have very low for nominal operation is for the worst-case condition of
shorted output. The diode current will then increase to the
typical peak switch current. Peak reverse voltage is equal
to the regulator input voltage. Use a diode with a reverse
voltage rating greater than the input voltage.

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LT3991/LT3991-3.3/LT3991-5
Applications Information
Table 4. Schottky Diodes. The Reverse Current Values Listed Are Schottky diodes often have larger forward voltage drops
Estimates Based Off of Typical Curves for Reverse Current at a given current, so a trade-off can exist between low
vs Reverse Voltage at 25°C.
load and high load efficiency. Often Schottky diodes with
IR at VR =
VR IAVE VF at 1A VF at 2A 20V 25°C larger reverse bias ratings will have less leakage at a given
PART NUMBER (V) (A) (mV) (mV) (µA) output voltage than a diode with a smaller reverse bias
On Semiconductor rating. Therefore, superior leakage performance can be
MBR0520L 20 0.5 30 achieved at the expense of diode size. Table 4 lists several
MBR0540 40 0.5 620 0.4 Schottky diodes and their manufacturers.
MBRM120E 20 1 530 595 0.5
MBRM140 40 1 550 20 Ceramic Capacitors
Diodes Inc. Ceramic capacitors are small, robust and have very low
B0530W 30 0.5 15 ESR. However, ceramic capacitors can cause problems
B0540W 40 0.5 620 1 when used with the LT3991 due to their piezoelectric nature.
B120 20 1 500 1.1 When in Burst Mode operation, the LT3991’s switching
B130 30 1 500 1.1 frequency depends on the load current, and at very light
B140 40 1 500 1.1 loads the LT3991 can excite the ceramic capacitor at audio
B150 50 1 700 0.4 frequencies, generating audible noise. Since the LT3991
B220 20 2 500 20 operates at a lower current limit during Burst Mode op-
B230 30 2 500 0.6 eration, the noise is typically very quiet to a casual ear. If
B140HB 40 1 1 this is unacceptable, use a high performance tantalum or
DFLS240L 40 2 500 4 electrolytic capacitor at the output.
DFLS140 40 1.1 510 1 A final precaution regarding ceramic capacitors concerns
DFLS160 60 1 500 2.5 the maximum input voltage rating of the LT3991. As pre-
DFLS2100 100 2 770 860 0.01 viously mentioned, a ceramic input capacitor combined
B240 40 2 500 0.45 with trace or cable inductance forms a high quality (under
Central Semiconductor damped) tank circuit. If the LT3991 circuit is plugged into a
CMSH1 - 40M 40 1 500 live supply, the input voltage can ring to twice its nominal
CMSH1 - 60M 60 1 700 value, possibly exceeding the LT3991’s rating. This situation
CMSH1 - 40ML 40 1 400 is easily avoided (see the Hot Plugging Safely section).
CMSH2 - 40M 40 2 550
CMSH2 - 60M 60 2 700 BOOST and BD Pin Considerations
CMSH2 - 40L 40 2 400 Capacitor C3 and the internal boost Schottky diode (see
CMSH2 - 40 40 2 500 the Block Diagram) are used to generate a boost volt-
CMSH2 - 60M 60 2 700 age that is higher than the input voltage. In most cases
An additional consideration is reverse leakage current. a 0.47μF capacitor will work well. Figure 3 shows three
When the catch diode is reversed biased, any leakage ways to arrange the boost circuit. The BOOST pin must
current will appear as load current. When operating under be more than 2.3V above the SW pin for best efficiency.
light load conditions, the low supply current consumed For outputs of 3V and above, the standard circuit (Figure 3a)
by the LT3991 will be optimized by using a catch diode is best. For outputs between 2.8V and 3V, use a 1μF boost
with minimum reverse leakage current. Low leakage capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage

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 LT3991/LT3991-3.3/LT3991-5
Applications Information
while using the internal boost diode. For reliable BOOST pin the maximum duty cycle as outlined in the Input Voltage
operation with 2.5V outputs use a good external Schottky Range section. For proper start-up, the minimum input
diode (such as the ON Semi MBR0540), and a 1μF boost voltage is also limited by the boost circuit. If the input
capacitor (Figure 3b). For output voltages below 2.5V, voltage is ramped slowly, the boost capacitor may not
the boost diode can be tied to the input (Figure 3c), or to be fully charged. Because the boost capacitor is charged
another external supply greater than 2.8V. However, the with the energy stored in the inductor, the circuit will rely
circuit in Figure 3a is more efficient because the BOOST pin on some minimum load current to get the boost circuit
current comes from a lower voltage source. You must also running properly. This minimum load will depend on input
be sure that the maximum voltage ratings of the BOOST and output voltages, and on the arrangement of the boost
and BD pins are not exceeded. circuit. The minimum load generally goes to zero once the
circuit has started. Figure 4 shows a plot of minimum load
The minimum operating voltage of an LT3991 application
to start and to run as a function of input voltage. In many
is limited by the minimum input voltage (4.3V) and by
cases the discharged output capacitor will present a load
to the switcher, which will allow it to start. The plots show
the worst-case situation where VIN is ramping very slowly.
BD
VIN VIN BOOST
For lower start-up voltage, the boost diode can be tied to
LT3991 C3
VIN; however, this restricts the input range to one-half of
the absolute maximum rating of the BOOST pin.
4.7µF SW VOUT
GND
5.0
4.8
4.6

(3a) For VOUT > 2.8V 4.4

INPUT VOLTAGE (V)

TO START
4.2
4.0
TO RUN
BD D2 3.8
VIN VIN BOOST 3.6
LT3991 C3 3.4 VOUT = 3.3V
TA = 25°C
3.2 L = 10µH
4.7µF SW VOUT f = 400kHz
GND 3.0
10 100 1000
LOAD CURRENT (mA)

6.4
(3b) For 2.5V < VOUT < 2.8V
6.2

6.0 TO START
INPUT VOLTAGE (V)

BD
VIN 5.8
VIN BOOST
LT3991 C3 5.6
TO RUN
4.7µF SW VOUT 5.4
GND VOUT = 5V
TA = 25°C
5.2 L = 10µH
3991 FO3
f = 400kHz
5.0
10 100 1000
(3c) For VOUT < 2.5V; VIN(MAX) = 27V LOAD CURRENT (mA)
3991 F04

Figure 3. Three Circuits for Generating the Boost Voltage Figure 4. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
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LT3991/LT3991-3.3/LT3991-5
Applications Information
At light loads, the inductor current becomes discontinu- Be aware that when the input voltage is below 4.3V, the
ous and this reduces the minimum input voltage to ap- input current may rise to several hundred μA. And the part
proximately 400mV above VOUT. At higher load currents, may be able to switch at cold or for VIN(EN) thresholds less
the inductor current is continuous and the duty cycle is than 7V. Figure 6 shows the magnitude of the increased
limited by the maximum duty cycle of the LT3991, requiring input current in a typical application with different pro-
a higher input voltage to maintain regulation. grammed VIN(EN).
When operating in Burst Mode for light load currents, the
Enable Pin
current through the VIN(EN) resistor network can easily be
The LT3991 is in shutdown when the EN pin is low and greater than the supply current consumed by the LT3991.
active when the pin is high. The rising threshold of the EN Therefore, the VIN(EN) resistors should be large to minimize
comparator is 1.01V, with 30mV of hysteresis. The EN pin their effect on efficiency at low loads.
can be tied to VIN if the shutdown feature is not used.
12V VIN(EN) Input Current
Adding a resistor divider from VIN to EN programs the 500
LT3991 to regulate the output only when VIN is above a
desired voltage (see Figure 5). Typically, this threshold, 400
VIN(EN), is used in situations where the input supply is cur-
rent limited, or has a relatively high source resistance. A INPUT CURRENT (µA) 300

switching regulator draws constant power from the source,

so source current increases as source voltage drops. This 200

looks like a negative resistance load to the source and can

cause the source to current limit or latch low under low 100

source voltage conditions. The VIN(EN) threshold prevents

0
the regulator from operating at source voltages where the 0 1 2 3 4 5 6 7 8 9 10 11 12
problems might occur. This threshold can be adjusted by INPUT VOLTAGE (V)
VIN(EN) = 12V
setting the values R3 and R4 such that they satisfy the R3 = 11M
R4 = 1M
following equation:
R3 6V VIN(EN) Input Current
VIN(EN) = +1 500
R4
where output regulation should not start until VIN is above 400
VIN(EN). Due to the comparator’s hysteresis, regulation will
INPUT CURRENT (µA)

not stop until the input falls slightly below VIN(EN). 300

200
LT3991
VIN

R3 100
1V +
EN SHDN
– 0
R4 0 1 2 3 4 5 6
3991 F05
INPUT VOLTAGE (V)
VIN(EN) = 6V 3991 F06

R3 = 5M
Figure 5. Programmed Enable Threshold R4 = 1M

Figure 6. Input Current vs Input Voltage

for a Programmed VIN(EN) of 6V and 12V

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 LT3991/LT3991-3.3/LT3991-5
Applications Information
Soft-Start The LT3991 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
The SS pin can be used to soft-start the LT3991 by throttling
instead will pulse skip to maintain regulation.
the maximum input current during start-up. An internal 1μA
current source charges an external capacitor generating a The LT3991 may be synchronized over a 250kHz to 2MHz
voltage ramp on the SS pin. The SS pin clamps the internal range. The RT resistor should be chosen to set the LT3991
VC node, which slowly ramps up the current limit. Maximum switching frequency 20% below the lowest synchronization
current limit is reached when the SS pin is about 1.5V or input. For example, if the synchronization signal will be
higher. By selecting a large enough capacitor, the output 250kHz and higher, the RT should be selected for 200kHz.
can reach regulation without overshoot. A 100k resistor To assure reliable and safe operation the LT3991 will only
in series with the soft-start capacitor is recommended. synchronize when the output voltage is near regulation as
Figure 7 shows start-up waveforms for a typical application indicated by the PG flag. It is therefore necessary to choose
with a 10nF capacitor and a 100k resistor on SS for a 3.3Ω a large enough inductor value to supply the required output
load when the EN pin is pulsed high for 10ms. current at the frequency set by the RT resistor (see the
Inductor Selection section). The slope compensation is set
The external SS capacitor is only actively discharged when
by the RT value, while the minimum slope compensation
EN is low. With EN low, the external SS cap is discharged
required to avoid subharmonic oscillations is established
through approximately 150Ω. The EN pin needs to be low
by the inductor size, input voltage, and output voltage.
long enough for the external cap to completely discharge
Since the synchronization frequency will not change the
through the 150Ω pull-down and external series resistor
slopes of the inductor current waveform, if the inductor
prior to start-up.
is large enough to avoid subharmonic oscillations at the
frequency set by RT, than the slope compensation will be
VSS
sufficient for all synchronization frequencies.
0.5V/DIV
Shorted and Reversed Input Protection
VOUT
2V/DIV If the inductor is chosen so that it won’t saturate exces-
sively, an LT3991 buck regulator will tolerate a shorted
IL output. There is another situation to consider in systems
0.5A/DIV where the output will be held high when the input to the
2ms/DIV 3991 F07
LT3991 is absent. This may occur in battery charging ap-
Figure 7. Soft-Start Waveforms for Front-Page Application plications or in battery backup systems where a battery
with 10nF Capacitor and 100k Series Resistor on SS. or some other supply is diode ORed with the LT3991’s
EN is Pulsed High for About 10ms with a 3.3Ω Load Resistor output. If the VIN pin is allowed to float and the EN pin
is held high (either by a logic signal or because it is tied
Synchronization to VIN), then the LT3991’s internal circuitry will pull its
To select low ripple Burst Mode operation, tie the SYNC pin quiescent current through its SW pin. This is fine if your
below 0.6V (this can be ground or a logic low output). system can tolerate a few μA in this state. If you ground
the EN pin, the SW pin current will drop to essentially
Synchronizing the LT3991 oscillator to an external fre-
zero. However, if the VIN pin is grounded while the output
quency can be done by connecting a square wave (with
is held high, regardless of EN, parasitic diodes inside the
20% to 80% duty cycle) to the SYNC pin. The square
LT3991 can pull current from the output through the SW
wave amplitude should have valleys that are below 0.6V
pin and the VIN pin. Figure 8 shows a circuit that will run
and peaks above 1.0V (up to 6V).
only when the input voltage is present and that protects
against a shorted or reversed input.

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17
LT3991/LT3991-3.3/LT3991-5
Applications Information
D4
MBRS140
VIN VIN BOOST L1
C2
VOUT

EN SW VOUT

LT3991

BD
GND FB
+
BACKUP
RPG GND
C3 RT
C4
3991 F07

Figure 8. Diode D4 Prevents a Shorted Input from Discharging a R2

Backup Battery Tied to the Output. It Also Protects the Circuit from C5 R1
a Reversed Input. The LT3991 Runs Only When the Input is Present
D1 C1
PCB Layout
GND
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 9 shows the 3991 F09

recommended component placement with trace, ground VIAS TO LOCAL GROUND PLANE VIAS TO RUN/SS VIAS TO VIN

plane and via locations. Note that large, switched currents VIAS TO VOUT VIAS TO SYNC VIAS TO PG OUTLINE OF LOCAL
GROUND PLANE
flow in the LT3991’s VIN and SW pins, the catch diode
(D1), and the input capacitor (C1). The loop formed by Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
these components should be as small as possible. These
components, along with the inductor and output capacitor,
the VIN pin of the LT3991 can ring to twice the nominal
should be placed on the same side of the circuit board,
input voltage, possibly exceeding the LT3991’s rating and
and their connections should be made on that layer. Place
damaging the part. If the input supply is poorly controlled
a local, unbroken ground plane below these components.
or the user will be plugging the LT3991 into an energized
The SW and BOOST nodes should be as small as possible.
supply, the input network should be designed to prevent
Finally, keep the FB and RT nodes small so that the ground
this overshoot. See Linear Technology Application Note
traces will shield them from the SW and BOOST nodes.
88 for a complete discussion.
The Exposed Pad on the bottom of the package must be
soldered to ground so that the pad acts as a heat sink. To High Temperature Considerations
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near For higher ambient temperatures, care should be taken in
the LT3991 to additional ground planes within the circuit the layout of the PCB to ensure good heat sinking of the
board and on the bottom side. LT3991. The Exposed Pad on the bottom of the package
must be soldered to a ground plane. This ground should be
Hot Plugging Safely tied to large copper layers below with thermal vias; these
layers will spread heat dissipated by the LT3991. Placing
The small size, robustness and low impedance of ceramic
additional vias can reduce thermal resistance further. The
capacitors make them an attractive option for the input
maximum load current should be derated as the ambient
bypass capacitor of LT3991 circuits. However, these ca-
temperature approaches the maximum junction rating.
pacitors can cause problems if the LT3991 is plugged into
a live supply. The low loss ceramic capacitor, combined Power dissipation within the LT3991 can be estimated by
with stray inductance in series with the power source, calculating the total power loss from an efficiency measure-
forms an under damped tank circuit, and the voltage at ment and subtracting the catch diode loss and inductor
3991fa

18
 LT3991/LT3991-3.3/LT3991-5
Applications Information
loss. The die temperature is calculated by multiplying the avoid excessive increase in light load supply current at
LT3991 power dissipation by the thermal resistance from high temperatures.
junction to ambient.
Other Linear Technology Publications
Also keep in mind that the leakage current of the power
Schottky diode goes up exponentially with junction tem- Application Notes 19, 35 and 44 contain more detailed
perature. When the power switch is closed, the power descriptions and design information for buck regulators
Schottky diode is in parallel with the power converter’s and other switching regulators. The LT1376 data sheet
output filter stage. As a result, an increase in a diode’s has a more extensive discussion of output ripple, loop
leakage current results in an effective increase in the load, compensation and stability testing. Design Note 318
and a corresponding increase in input power. Therefore, shows how to generate a bipolar output supply using a
the catch Schottky diode must be selected with care to buck regulator.

Typical Applications
5V Step-Down Converter 2.5V Step-Down Converter
VIN VIN
6.6V TO 55V 4.3V TO 55V

VIN VIN

OFF ON EN BOOST OFF ON EN BOOST

0.47µF 1µF
PG 10µH PG 10µH
4.7µF SW 4.7µF SW
SS LT3991 SS LT3991

RT RT
BD BD
10pF 10pF

118k 1M VOUT 162k 1M VOUT

FB 5V FB 2.5V
SYNC GND SYNC GND 1.2A
1.2A
309k 47µF 909k 47µF
f = 400kHz f = 300kHz

3991 TA02 3991 TA03

3.3V Step-Down Converter 5V Step-Down Converter

VIN VIN
4.3V TO 55V 6.6V TO 55V
VIN VIN
OFF ON EN BOOST OFF ON EN./UVLO BOOST
0.47µF 10µH 0.47µF 15µH
PG VOUT PG
SW 3.3V SW
SS LT3991 1.2A SS LT3991-5
4.7µF 4.7µF
RT RT
BD BD VOUT
10pF
118k VOUT 5V
SYNC GND 1.2A
1.78M 47µF
118k FB
SYNC GND 3991 TA10
1M 47µF f = 400kHz

3991 TA09
f = 400kHz

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19
LT3991/LT3991-3.3/LT3991-5
Typical Applications
1.8V Step-Down Converter
VIN
4.3V TO 37V

VIN BD
OFF ON EN BOOST
0.47µF
PG 6.8µH
SW
SS LT3991
4.7µF
RT
10pF

162k 511k VOUT

FB 1.8V
SYNC GND 1.2A
1M 100µF
f = 300kHz
3991 TA05

12V Step-Down Converter

VIN
16V TO 55V

VIN
OFF ON EN BOOST
0.47µF
PG
10µF SW
SS LT3991 10µH

RT
BD
10pF

1M VOUT
49.9k FB 12V
SYNC GND 1.2A
110k 10µF
f = 800kHz
3991 TA06

3.3V Step-Down Converter with Undervoltage Lockout, Soft-Start, and Power Good
VIN
6V TO 55V
5M VIN BOOST
EN 0.47µF
10µH
SW

4.7µF SS LT3991 150k

100k RT PG PGOOD

1M BD
10pF
1nF
1M VOUT
118k FB 3.3V
SYNC GND 1.2A
562k 47µF
f = 400kHz
3991 TA06

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20
 LT3991/LT3991-3.3/LT3991-5
Typical Applications
4V Step-Down Converter with a High Impedance Input Source

+
10M VIN
48V
– EN BOOST
+ CBULK 432k 0.47µF
100µF PG 10µH * AVERAGE OUTPUT POWER CANNOT
SW EXCEED THAT WHICH CAN BE PROVIDED
SS LT3991 BY HIGH IMPEDANCE SOURCE.
4.7µF NAMELY,
100k RT V2
POUT(MAX) = •η
BD 4R
2nF 10pF
WHERE V IS VOLTAGE OF SOURCE, R IS
VOUT INTERNAL SOURCE IMPEDANCE, AND η IS
118k 1M 4V LT3971 EFFICIENCY. MAXIMUM OUTPUT
FB 1.2A* CURRENT OF 1.2A CAN BE SUPPLIED FOR A
SYNC GND
SHORT TIME BASED ON THE ENERGY
412k 100µF
f = 400kHz WHICH CAN BE SOURCED BY THE BULK
INPUT CAPACITANCE.
3991 TA07a

Sourcing a Maximum Load Pulse Start-Up from High Impedance Input Source

VIN VIN
10V/DIV 2V/DIV

VOUT
200mV/DIV VOUT
2V/DIV

IL IL
1A/DIV 1A/DIV

3991 TA07c
500µs/DIV 3991 TA07b 2ms/DIV

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21
LT3991/LT3991-3.3/LT3991-5
Package Description
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699 Rev C)
R = 0.125 0.40 ± 0.10
TYP
6 10
0.70 ±0.05

3.55 ±0.05 1.65 ±0.05 3.00 ±0.10 1.65 ± 0.10

2.15 ±0.05 (2 SIDES) (4 SIDES) (2 SIDES) PIN 1 NOTCH
PIN 1 R = 0.20 OR
PACKAGE TOP MARK 0.35 × 45°
OUTLINE (SEE NOTE 6) CHAMFER
(DD) DFN REV C 0310

5 1
0.25 ± 0.05 0.200 REF 0.75 ±0.05 0.25 ± 0.05
0.50 0.50 BSC
BSC 2.38 ±0.10
2.38 ±0.05 (2 SIDES)
0.00 – 0.05
(2 SIDES) BOTTOM VIEW—EXPOSED PAD
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
NOTE: 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 5. EXPOSED PAD SHALL BE SOLDER PLATED
2. DRAWING NOT TO SCALE 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
3. ALL DIMENSIONS ARE IN MILLIMETERS TOP AND BOTTOM OF PACKAGE

MSE Package
10-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1664 Rev G)

BOTTOM VIEW OF
EXPOSED PAD OPTION
1.88
1.88 ± 0.102
(.074 ± .004) 0.889 ± 0.127 1 (.074) 0.29
(.035 ± .005) 1.68 REF
(.066)

5.23 0.05 REF

(.206) 1.68 ± 0.102 3.20 – 3.45
(.066 ± .004) (.126 – .136) DETAIL “B”
MIN CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
10
NO MEASUREMENT PURPOSE
0.50 3.00 ± 0.102
0.305 ± 0.038 (.0197) (.118 ± .004) 0.497 ± 0.076
(.0120 ± .0015) BSC
TYP (NOTE 3) (.0196 ± .003)
10 9 8 7 6
RECOMMENDED SOLDER PAD LAYOUT REF

4.90 ± 0.152 3.00 ± 0.102

(.193 ± .006) (.118 ± .004)
DETAIL “A”
0.254 (NOTE 4)
(.010)
0° – 6° TYP
GAUGE PLANE
1 2 3 4 5
0.53 ± 0.152 1.10 0.86
(.021 ± .006) (.043) (.034)
MAX REF
DETAIL “A”
0.18
(.007)
SEATING
PLANE 0.17 – 0.27 0.1016 ± 0.0508
(.007 – .011) (.004 ± .002)
0.50
TYP MSOP (MSE) 0910 REV G
NOTE: (.0197)
1. DIMENSIONS IN MILLIMETER/(INCH) BSC
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD
SHALL NOT EXCEED 0.254mm (.010") PER SIDE.
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22
 LT3991/LT3991-3.3/LT3991-5
Revision History
REV DATE DESCRIPTION PAGE NUMBER
A 01/11 Added 3.3V and 5V fixed voltage options reflected throughout the data sheet. 1-24

3991fa

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. 23
LT3991/LT3991-3.3/LT3991-5
Related Parts
PART DESCRIPTION COMMENTS
LT3970 40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC VIN = 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD <1 µA,
Converter with IQ = 2.5µA 3mm × 2mm DFN-10, MSOP-10 Packages
LT3990 62V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC VIN = 4.2V to 62V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD <1 µA,
Converter with IQ = 2.5µA 3mm × 2mm DFN-10, MSOP-10 Packages
LT3971 38V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC VIN = 4.3V to 38V, VOUT(MIN) = 1.21V, IQ = 2.8µA, ISD <1 µA,
Converter with IQ = 2.8µA 3mm × 3mm DFN-10, MSOP-10E Packages
LT3682 36V, 60V Max, 1A, 2.2MHz High Efficiency Micropower Step-Down VIN = 3.6V to 36V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD <1 µA,
DC/DC Converter 3mm × 3mm DFN-12 Package
LT3689 36V, 60V Transient Protection, 800mA, 2.2MHz High Efficiency VIN = 3.6V to 36V (Transient to 60V), VOUT(MIN) = 0.8V, IQ = 75µA,
Micropower Step-Down DC/DC Converter with POR Reset and ISD <1 µA, 3mm × 3mm QFN-16 Package
Watchdog Timer
LT3480 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High VIN = 3.6V to 36V (Transient to 60V), VOUT(MIN) = 0.78V, IQ = 70µA,
Efficiency Step-Down DC/DC Converter with Burst Mode Operation ISD <1 µA, 3mm × 3mm DFN-10, MSOP-10E Packages
LT3980 58V with Transient Protection to 80V, 2A (IOUT), 2.4MHz, High VIN = 3.6V to 58V (Transient to 60V), VOUT(MIN) = 0.78V, IQ = 85µA,
Efficiency Step-Down DC/DC Converter with Burst Mode Operation ISD <1 µA, 3mm × 4mm DFN-16, MSOP-16E Packages

3991fa

24 Linear Technology Corporation

LT 0211 REV A • PRINTED IN USA

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com  LINEAR TECHNOLOGY CORPORATION 2009