RT8272

3A, 24V, 1.2MHz Step-Down Converter

General Description Features
The RT8272 is a high voltage buck converter that can z Wide Operating Input Range : 4.75V to 24V
support the input voltage range from 4.75V to 24V and the z Adjustable Output Voltage Range : 0.92V to 15V
output current can be up to 3A. Current Mode operation z Output Current up to 3A
provides fast transient response and eases loop z μA Low Shutdown Current
25μ
stabilization. z Power MOSFET : 0.1Ω Ω
z High Efficiency up to 95%
The chip provides protection functions such as cycle-by-
z 1.2MHz Fixed Switching Frequency
cycle current limiting and thermal shutdown protection.
z Stable with Low ESR Output Ceramic Capacitors
In shutdown mode, the regulator draws 25μA of supply
z Thermal Shutdown Protection
current. The RT8272 is available in a SOP-8 (Exposed
z Cycle-By-Cycle Over Current Protection
Pad) surface mount package.
z RoHS Compliant and Halogen Free

Ordering Information
RT8272
Applications
Package Type z Distributive Power Systems
SP : SOP-8 (Exposed Pad-Option 1) z Battery Charger
Lead Plating System z DSL Modems
G : Green (Halogen Free and Pb Free) z Pre-regulator for Linear Regulators
Note :
Richtek products are : Pin Configurations
` RoHS compliant and compatible with the current require- (TOP VIEW)
ments of IPC/JEDEC J-STD-020.
BOOT 8 SS
` Suitable for use in SnPb or Pb-free soldering processes. VIN 2 7 EN
GND
SW 3 6 COMP
9
GND 4 5 FB

SOP-8 (Exposed Pad)

Typical Application Circuit

VIN 2 1
VIN BOOT
4.75V to 24V CIN CBOOT
RT8272 L
10µF 10nF 4.7µH VOUT
Chip Enable SW 3
7 EN D1 3.3V/3A
B330 R1
25.8k
8 SS COUT
CSS FB 5 47µF
4, CC RC
0.1µF R2
9 (Exposed Pad) 2.2nF 39k 10k
GND 6
COMP

CP
NC

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RT8272
Table 1. Recommended Component Selection
VOUT (V) R1 (kΩ) R2 (kΩ) R C (kΩ) C C (nF) C OUT (μF) L (μH)
15 153 10 82 0.82 47 10
10 100 10 62 1.2 47 10
8 77 10 62 1.5 47 6.8
5 43 10 41 2.2 47 6.8
3.3 25.8 10 39 2.2 47 4.7
2.5 17 10 21 2.2 47 4.7
1.8 9.1 10 15 2.2 47 2.2
1.2 3 10 15 2.2 47 2.2

Functional Pin Description

Pin No. Pin Name Pin Function
High Side Gate Drive Boost Input. BOOT supplies the drive for the high side
1 BOOT N-MOSFET switch. Connect a 10nF or greater capacitor from SW to BOOT to
power the high side switch.
Power Input. V IN supplies the power to the IC, as well as the step-down converter
2 VIN switches. Bypass VIN to GND with a suitable large capacitor to eliminate noise on
the input to the IC.
Power Switching Output. SW is the switching node that supplies power to the
3 SW output. Connect the output LC filter from SW to the output load. Note that a
capacitor is required from SW to BOOT to power the high side switch.
4, Ground. The exposed pad must be soldered to a large PCB and connected to
GND
9 (Exposed Pad) GND for maximum power dissipation.
Feedback Input. FB senses the output voltage to regulate said voltage. The
5 FB
feedback reference voltage is 0.92V typically.
Compensation Node. COMP is used to compensate the regulation control loop.
Connect a series RC network from COMP to GND to compensate the regulation
6 COMP
control loop. In some cases, an additional capacitor from COMP to GND is
required.
Enable Input. EN is a digital input that turns the regulator on or off. Drive EN
7 EN higher than 1.4V to turn on the regulator, lower than 0.4V to turn it off. If the EN
pin is open, it will be pulled to high by internal circuit.
Soft-Start Control Input. SS controls the soft start period. Connect a capacitor
8 SS from SS to GND to set the soft-start period. A 0.1μF capacitor sets the soft-start
period to 10ms.

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 RT8272
Function Block Diagram
VIN
VCC Current Sense
Internal Oscillator Slope Comp Amplifier
Regulator 1.2MHz/440kHz + VA
1µA -
VA VCC Foldback
10k
EN - Control BOOT
1V +
0.5V +
Shutdown SW
3V - Logic
Comparator +
UV
VCC Comparator -
Current GND
10µA Comparator
0.92V +
SS +EA
-
Gm = 680µA/V

FB COMP

Absolute Maximum Ratings (Note 1)

z Supply Voltage, VIN ----------------------------------------------------------------------------------------- −0.3V to 26V
z Switching Voltage, SW ------------------------------------------------------------------------------------- −0.3V to (VIN + 0.3V)
z BOOT Voltage ------------------------------------------------------------------------------------------------ (VSW − 0.3V) to (VSW + 6V)
z All Other Pins Voltage -------------------------------------------------------------------------------------- −0.3V to 6V
z Power Dissipation, PD @ TA = 25°C
SOP-8 (Exposed Pad) -------------------------------------------------------------------------------------- 1.333W
z Package Thermal Resistance (Note 2)
SOP-8 (Exposed Pad), θJA -------------------------------------------------------------------------------- 75°C/W
z Junction Temperature --------------------------------------------------------------------------------------- 150°C
z Lead Temperature (Soldering, 10 sec.) ----------------------------------------------------------------- 260°C
z Storage Temperature Range ------------------------------------------------------------------------------- −65°C to 150°C
z ESD Susceptibility (Note 3)
HBM (Human Body Mode) --------------------------------------------------------------------------------- 2kV
MM (Machine Mode) ---------------------------------------------------------------------------------------- 200V

Recommended Operating Conditions (Note 4)

z Supply Voltage, VIN ----------------------------------------------------------------------------------------- 4.75V to 24V
z Enable Voltage, VEN ----------------------------------------------------------------------------------------- 0V to 5.5V
z Junction Temperature Range ------------------------------------------------------------------------------ −40°C to 125°C
z Ambient Temperature Range ------------------------------------------------------------------------------ −40°C to 85°C

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RT8272
Electrical Characteristics
(VIN = 12V, TA = 25°C unless otherwise specified)

Parameter Symbol Test Conditions Min Typ Max Unit

Feedback Reference Voltage VFB 4.75V ≤ VIN ≤ 24V 0.902 0.92 0.938 V
High Side Switch-On Resistance RDS(ON)1 -- 0.1 0.16 Ω
Low Side Switch-On Resistance RDS(ON)2 -- 10 -- Ω
Switch Leakage VEN = 0V, VSW = 0V -- -- 10 μA
Current Limit ILIM Duty = 80%; VBOOT−SW = 4.8V -- 4.1 -- A
Current Sense Transconductance GCS Output Current to VCOMP -- 4 -- A/V
Error Amplifier Tansconductance Gm ΔIC = ±10μA 500 680 900 μA/V
Oscillator Frequency fSW -- 1.2 -- MHz
Short Circuit Oscillation Frequency VFB = 0V -- 440 -- kHz
Maximum Duty Cycle DMAX VFB = 0.8V -- 80 -- %
Minimum On-Time tON -- 90 -- ns
Under Voltage Lockout Threshold
Rising 3.8 4.2 4.5 V
Under Voltage Lockout Threshold
Hysteresis -- 300 -- mV
En input Low Voltage -- -- 0.4 V
En input High Voltage 1.4 -- -- V
Enable Pull Up Current 0.15 1 2.65 μA
Shutdown Current ISHDN VEN = 0V -- 25 -- μA
Quiescent Current IQ VEN = 2V, VFB = 1V -- 0.8 1 mA
Soft-Start Current ISS -- 10 -- μA
Soft-Start Period CSS = 0.1μF -- 10 -- ms
Thermal Shutdown T SD -- 150 -- °C

Note 1. Stresses listed as the above "Absolute Maximum Ratings" may cause permanent damage to the device. These are for
stress ratings. Functional operation of the device at these or any other conditions beyond those indicated in the
operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended
periods may remain possibility to affect device reliability.
Note 2. θJA is measured in the natural convection at TA = 25°C on a high effective four layers thermal conductivity test board of
JEDEC 51-7 thermal measurement standard.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.

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 RT8272
Typical Operating Characteristics
Efficiency vs. Output Current Reference Voltage vs. Input Voltage
100 0.925
90 VIN = 12V 0.924
80 0.923

Reference Voltage (V)

VIN = 24V
70 0.922
Efficiency (%)

60 0.921
50 0.920
40 0.919
30 0.918
20 0.917
10 0.916
VOUT = 3.3V
0 0.915
0 0.5 1 1.5 2 2.5 3 4 6 8 10 12 14 16 18 20 22 24
Output Current (A) Input Voltage (V)

Reference Voltage vs. Temperature Output Voltage vs. Output Current

0.94 3.350

0.935 3.345
Reference Voltage (V)

3.340
Output Voltage (V)

0.93
3.335
0.925 3.330
0.92 3.325

0.915 3.320
3.315
0.91
3.310
0.905 3.305
VIN = 12V VIN = 12V
0.9 3.300
-50 -25 0 25 50 75 100 125 0 0.5 1 1.5 2 2.5 3
Temperature (°C) Output Current (A)

Frequency vs. Input Voltage Frequency vs. Temperature

1.23 1.30

1.22 1.25
Frequency (MHz)1
Frequency (MHz)1

1.21 1.20

1.20 1.15

1.19 1.10

1.18 1.05
VOUT = 3.3V, IOUT = 0.3A VIN = 12V, VOUT = 3.3V, IOUT = 0.3A
1.17 1.00
4 6 8 10 12 14 16 18 20 22 24 -50 -25 0 25 50 75 100 125
Input Voltage (V) Temperature (°C)

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RT8272

Current Limit vs. Duty Cycle Quiescent Current vs. Temperature

6.2 0.90

5.8 0.85

Quiescent Current (mA)

5.4
0.80
Current Limit (A)

5.0
0.75
4.6
0.70
4.2
0.65
3.8

3.4 0.60
VIN = 12V, VOUT = 3.3V
3.0 0.55
10 20 30 40 50 60 70 80 -50 -25 0 25 50 75 100 125
Duty Cycle (%) Temperature (°C)

UVLO vs. Temperature Enable Threshold vs. Temperature

5.0 1.40
4.8 1.30
4.6 1.20
Enable Threshold (V)

Rising
4.4 1.10
UVLO (V)

4.2 1.00
Rising
4.0 0.90 Falling
3.8 0.80
3.6 Falling 0.70
3.4 0.60
3.2 0.50
3.0 0.40
-50 -25 0 25 50 75 100 125 -50 -25 0 25 50 75 100 125
Temperature (°C) Temperature (°C)

Load Transient Response Load Transient Response

VIN = 12V, VOUT = 3.3V VIN = 12V, VOUT = 3.3V
IOUT = 1.5A to 3A IOUT = 0A to 3A
VOUT VOUT
(50mV/Div) (100mV/Div)

IOUT IOUT
(2A/Div) (2A/Div)

Time (100μs/Div) Time (100μs/Div)

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 RT8272

Power On from EN Pin Power Off from EN Pin

VIN = 12V, VOUT = 3.3V
IOUT = 3A
VEN VEN
(5V/Div) (5V/Div)

VOUT VOUT
(1V/Div) (1V/Div)
I IN VIN = 12V, VOUT = 3.3V I IN
(500mA/Div) IOUT = 3A
(500mA/Div)

Time (2.5ms/Div) Time (100μs/Div)

Output Ripple

VOUT
(5mV/Div)

VLX
(10V/Div)

IL VIN = 12V, VOUT = 3.3V

(2A/Div)
IOUT = 3A

Time (500ns/Div)

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RT8272
Application Information
The RT8272 is an asynchronous high voltage buck Soft-Start
converter that can support the input voltage range from The RT8272 contains an external soft-start clamp that
4.75V to 24V and the output current can be up to 3A. gradually raises the output voltage. The soft-start timming
can be programed by the external capacitor between SS
Output Voltage Setting
pin and GND. The chip provides a 10μA charge current for
The resistive divider allows the FB pin to sense the output
the external capacitor. If 0.1μF capacitor is used to set
voltage as shown in Figure 1.
the soft-start and it’ s period will be 10ms(typ.).
VOUT
Inductor Selection
R1 The inductor value and operating frequency determine the
FB ripple current according to a specific input and output
RT8272 R2 voltage. The ripple current ΔIL increases with higher VIN
GND and decreases with higher inductance.
ΔIL = ⎡⎢ OUT ⎤⎥ × ⎡⎢1− OUT ⎤⎥
V V
Figure 1. Output Voltage Setting ⎣ f ×L ⎦ ⎣ VIN ⎦
Having a lower ripple current reduces not only the ESR
The output voltage is set by an external resistive divider
losses in the output capacitors but also the output voltage
according to the following equation :
ripple. High frequency with small ripple current can achieve
VOUT = VFB ⎛⎜ 1 + R1 ⎞⎟ highest efficiency operation. However, it requires a large
⎝ R2 ⎠
inductor to achieve this goal.
Where VFB is the feedback reference voltage (0.92V typ.).
For the ripple current selection, the value of ΔIL = 0.4(IMAX)
External Bootstrap Diode will be a reasonable starting point. The largest ripple
Connect a 10nF low ESR ceramic capacitor between the current occurs at the highest VIN. To guarantee that the
BOOT pin and SW pin. This capacitor provides the gate ripple current stays below the specified maximum, the
driver voltage for the high side MOSFET. inductor value should be chosen according to the following
equation :
It is recommended to add an external bootstrap diode
⎡ VOUT ⎤ ⎡ VOUT ⎤
between an external 5V and the BOOT pin for efficiency L =⎢ ⎥ × ⎢1− ⎥
⎣ f × ΔIL(MAX) ⎦ ⎣ VIN(MAX) ⎦
improvement when input voltage is lower than 5.5V or duty
ratio is higher than 65%. The bootstrap diode can be a Inductor Core Selection
low cost one such as 1N4148 or BAT54. The inductor type must be selected once the value for L
The external 5V can be a 5V fixed input from system or a is known. Generally speaking, high efficiency converters
5V output of the RT8272. can not afford the core loss found in low cost powdered
5V
iron cores. So, the more expensive ferrite or
mollypermalloy cores will be a better choice.
The selected inductance rather than the core size for a
BOOT fixed inductor value is the key for actual core loss. As the
RT8272 10nF inductance increases, core losses decrease. Unfortunately,
SW increase of the inductance requires more turns of wire
and therefore the copper losses will increase.
Figure 2. External Bootstrap Diode
Ferrite designs are preferred at high switching frequency
due to the characteristics of very low core losses. So,
design goals can focus on the reduction of copper loss
and the saturation prevention.
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 RT8272
Ferrite core material saturates “hard”, which means that refer to table 2 for more detail. The input capacitor has to
inductance collapses abruptly when the peak design connect another 10μF ceramic capacitor between the input
current is exceeded. The previous situation results in an and ground when the input voltage is lower than 6.5V.
abrupt increase in inductor ripple current and consequent The selection of COUT is determined by the required ESR
output voltage ripple. to minimize voltage ripple.
Do not allow the core to saturate! Moreover, the amount of bulk capacitance is also a key
Different core materials and shapes will change the size/ for COUT selection to ensure that the control loop is stable.
current and price/current relationship of an inductor. Loop stability can be checked by viewing the load transient
Toroid or shielded pot cores in ferrite or permalloy materials response as described in a later section.
are small and do not radiate energy. However, they are The output ripple, ΔVOUT , is determined by :

ΔVOUT ≤ ΔIL ⎡⎢ESR + ⎤

usually more expensive than the similar powdered iron 1
inductors. The rule for inductor choice mainly depends ⎣ 8fCOUT ⎥⎦
on the price vs. size requirement and any radiated field/ The output ripple will be highest at the maximum input
EMI requirements. voltage since ΔIL increases with input voltage. Multiple
Diode Selection capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirement. Dry tantalum,
When the power switch turns off, the path for the current
special polymer, aluminum electrolytic and ceramic
is through the diode connected between the switch output
capacitors are all available in surface mount packages.
and ground. This forward biased diode must have a
Special polymer capacitors offer very low ESR value.
minimum voltage drop and recovery times. Schottky diode
However, it provides lower capacitance density than other
is recommended and it should be able to handle those
types. Although Tantalum capacitors have the highest
current. The reverse voltage rating of the diode should be
capacitance density, it is important to only use types that
greater than the maximum input voltage, and current rating
pass the surge test for use in switching power supplies.
should be greater than the maximum load current. For
Aluminum electrolytic capacitors have significantly higher
more detail please refer to Table 3.
ESR. However, it can be used in cost-sensitive applications
CIN and COUT Selection for ripple current rating and long term reliability
The input capacitance, C IN, is needed to filter the considerations. Ceramic capacitors have excellent low
trapezoidal current at the source of the high side MOSFET. ESR characteristics but can have a high voltage coefficient
To prevent large ripple current, a low ESR input capacitor and audible piezoelectric effects. The high Q of ceramic
sized for the maximum RMS current should be used. The capacitors with trace inductance can also lead to significant
RMS current is given by : ringing.
VOUT VIN Higher values, lower cost ceramic capacitors are now
IRMS = IOUT(MAX) −1
VIN VOUT becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
This formula has a maximum at VIN = 2VOUT, where
for switching regulator applications. However, care must
I RMS = I OUT /2. This simple worst-case condition is
be taken when these capacitors are used at input and
commonly used for design because even significant
output. When a ceramic capacitor is used at the input
deviations do not offer much relief.
and the power is supplied by a wall adapter through long
Choose a capacitor rated at a higher temperature than wires, a load step at the output can induce ringing at the
required. Several capacitors may also be paralleled to input, VIN. At best, this ringing can couple to the output
meet size or height requirements in the design. and be mistaken as loop instability. At worst, a sudden
For the input capacitor, a 10μF low ESR ceramic capacitor inrush of current through the long wires can potentially
is recommended. For the recommended capacitor, please cause a voltage spike at VIN large enough to damage the
part.
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RT8272
Checking Transient Response
1.6
The regulator loop response can be checked by looking Four Layer PCB

Maximum Power Dissipation (W)

1.4
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When 1.2

a load step occurs, VOUT immediately shifts by an amount 1.0

equal to ΔILOAD (ESR) also begins to charge or discharge 0.8
COUT generating a feedback error signal for the regulator
0.6
to return VOUT to its steady-state value. During this recovery
time, VOUT can be monitored for overshoot or ringing that 0.4

would indicate a stability problem. 0.2

0.0
Thermal Considerations
0 25 50 75 100 125
For continuous operation, do not exceed the maximum Ambient Temperature (°C)
operation junction temperature 125°C. The maximum
power dissipation depends on the thermal resistance of Figure 3. Derating Curves for RT8272 Packages
IC package, PCB layout, the rate of surroundings airflow
and temperature difference between junction to ambient. Layout Consideration
The maximum power dissipation can be calculated by Follow the PCB layout guidelines for optimal performance
following formula : of RT8272.
PD(MAX) = ( TJ(MAX) - TA ) / θJA ` Keep the traces of the main current paths as short and
Where T J(MAX) is the maximum operation junction wide as possible.
temperature, TA is the ambient temperature and the θJA is ` Put the input capacitor as close as possible to the device
the junction to ambient thermal resistance. pins (VIN and GND).
For recommended operating conditions specification of ` LX node is with high frequency voltage swing and should
RT8272, where the maximum junction temperature is be kept at small area. Keep analog components away
125°C. The junction to ambient thermal resistance θJA is from the LX node to prevent stray capacitive noise pick-
layout dependent. For SOP-8 (Exposed Pad) packages, up.
the thermal resistance θJA is 75°C/W on the standard
` Connect feedback network behind the output capacitors.
JEDEC 51-7 four-layers thermal test board. The maximum
Keep the loop area small. Place the feedback
power dissipation at TA = 25°C can be calculated by
components near the RT8272.
following formula:
` Connect all analog grounds to a command node and
PD(MAX) = (125°C − 25°C) / (75°C/W) = 1.333W for
then connect the command node to the power ground
SOP-8 (Exposed Pad) packages
behind the output capacitors.
The maximum power dissipation depends on operating
` An example of PCB layout guide is shown in Figure 4
ambient temperature for fixed T J(MAX) and thermal
for reference.
resistance θJA. For RT8272 packages, the Figure 3 of
derating curves allows the designer to see the effect of
rising ambient temperature on the maximum power
allowed.

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 RT8272
GND VIN SW

CB CSS

CIN The parallel distance between

Input capacitor must be placed COMP and FB traces must be
BOOT 8 SS as short as possible.
as close to the IC as possible.
VIN 2 7 EN CC
D1 GND
SW 3 6 COMP
C OUT GND 4 5 FB
L CP RC
The output capacitor must be
placed near the RT8272. VOUT
GND
SW should be connected to inductor by
wide and short trace. Keep sensitive
components away from this trace.
VOUT
The resistor divider must be connected
as close to the device as possible.

Figure 4. PCB Layout Guide

Table 2. Suggested Inductors for Typical Application Circuit

Component Supplier Series Inductance (µH) DCR (mΩ) Current Rating (A) Dimensions (mm)

TDK RLF7030 4.7 31 3.5 7.3 x 6.8 x 3.2

TAIYO YUDEN NR8040 4.7 18 4.7 8x8x4
GOTERND GSSR2 4.7 18 5.7 10 x 10 x 3.8

Table 3. Suggested Capacitors for CIN and COUT

Component Supplier Part No. Capacitance (µF) Case Size
MURATA GRM31CR61E106K 10 1206
TDK C3225X5R1E106K 10 1206
TAIYO YUDEN TMK316BJ106ML 10 1206
MURATA GRM31CR60J476M 47 1206
TDK C3225X5R0J476M 47 1210
TAIYO YUDEN EMK325BJ476MM 47 1210

Table 4. Suggested Diode

Component Supplier Series VRRM (V) IOUT (A) Package
DIODES B330A 30 3 DO-214AC
DIODES B340 40 3 DO-214AB
PANJIT SK33 30 3 DO-214AB
PANJIT SK34 40 3 DO-214AB

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RT8272
Outline Dimension
H
A

EXPOSED THERMAL PAD Y

(Bottom of Package)
J X B

C
I
D

Dimensions In Millimeters Dimensions In Inches

Symbol
Min Max Min Max
A 4.801 5.004 0.189 0.197
B 3.810 4.000 0.150 0.157
C 1.346 1.753 0.053 0.069
D 0.330 0.510 0.013 0.020
F 1.194 1.346 0.047 0.053
H 0.170 0.254 0.007 0.010
I 0.000 0.152 0.000 0.006
J 5.791 6.200 0.228 0.244
M 0.406 1.270 0.016 0.050
X 2.000 2.300 0.079 0.091
Option 1
Y 2.000 2.300 0.079 0.091
X 2.100 2.500 0.083 0.098
Option 2
Y 3.000 3.500 0.118 0.138

8-Lead SOP (Exposed Pad) Plastic Package

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Headquarter Taipei Office (Marketing)
5F, No. 20, Taiyuen Street, Chupei City 5F, No. 95, Minchiuan Road, Hsintien City
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Email: marketing@richtek.com

Information that is provided by Richtek Technology Corporation is believed to be accurate and reliable. Richtek reserves the right to make any change in circuit
design, specification or other related things if necessary without notice at any time. No third party intellectual property infringement of the applications should be
guaranteed by users when integrating Richtek products into any application. No legal responsibility for any said applications is assumed by Richtek.

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