19-1175; Rev 0; 12/96

High-Accuracy Step-Down Controller

with Synchronous Rectifier for CPU Power
_______________General Description ____________________________Features

MAX798
The MAX798 † high-performance, step-down DC-DC ♦ 96% Efficiency
converter provides main CPU power in battery-powered ♦ 4.5V to 30V Input Range
systems. This buck controller achieves 96% efficiency
by using synchronous rectification and Maxim’s propri- ♦ 1.6V to 6V Adjustable Precision Output
etary Idle Mode™ control scheme to extend battery life ♦ ±0.4% Max Total Load-Regulation Error
at full-load (up to 10A) and no-load outputs. The ♦ 0.06%/V Max Line-Regulation Error
MAX798’s high accuracy meets the demanding require-
ments of the latest-generation CPUs. Excellent dynamic ♦ 5V Linear-Regulator Output
response corrects output transients caused by the latest ♦ Precision 2.505V Reference Output
dynamic-clock CPUs within five 300kHz clock cycles. ♦ Automatic Bootstrap Circuit
Unique bootstrap circuitry drives inexpensive N -channel
MOSFETs, reducing system cost and eliminating the crow - ♦ 150kHz/300kHz Fixed-Frequency PWM Operation
bar switching currents found in some PMOS/NMOS switch ♦ Programmable Soft-Start
designs. ♦ 1.2mA Typical Quiescent Current
The MAX798 has a logic-controlled and synchronizable (VIN = 12V, VOUT = 2.5V)
fixed-frequency pulse-width-modulating (PWM) operating ♦ 1µA Typical Shutdown Current
mode, which reduces noise and RF interference in sensi -
tive mobile-communications and pen-entry applications.
The SKIP override input allows automatic switchover to
idle-mode operation (for high-efficiency pulse skipping) at ______________Ordering Information
light loads, or forces fixed-frequency mode for lowest noise
PART TEMP. RANGE PIN-PACKAGE
at all loads.
MAX798ESE -40°C to +85°C 16 Narrow SO
________________________Applications
Notebook and Subnotebook Computers
PDAs and Mobile Communicators __________Typical Operating Circuit
INPUT
4.5V TO 30V
__________________Pin Configuration 5V, 25mA
4.7µF
TOP VIEW
V+ VL
BST
N
SS 1 16 DH DH
RSENSE
SKIP 2 15 LX VOUT
0.1µF
REF 3 14 BST MAX798
LX
GND 4 MAX798 13 DL
N
ON SHDN DL
SYNC 5 12 PGND OFF
SHDN 6 11 VL PGND

FB 7 10 V+ CSH
CSH 8 9 CSL CSL

SO
GND FB

Idle Mode is a trademark of Maxim Integrated Products.

†U

________________________________________________________________ Maxim Integrated Products 1

For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
 High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
ABSOLUTE MAXIMUM RATINGS
MAX798

V+ to GND .................................................................-0.3V, +36V REF Short Circuit to GND ...........................................Continuous

GND to PGND........................................................................±2V VL Output Current ...............................................................50mA
VL to GND ...................................................................-0.3V, +7V Continuous Power Dissipation (TA = +70°C)
BST to GND ...............................................................-0.3V, +36V SO (derate 8.70mW/°C above +70°C) ........................696mW
DH to LX ........................................................-0.3V, (BST + 0.3V) Operating Temperature Range
LX to BST.....................................................................-7V, +0.3V MAX798ESE....................................................-40°C to +85°C
SHDN to GND............................................................-0.3V, +36V Storage Temperature Range .............................-65°C to +160°C
SYNC, SS, REF, SKIP, DL to GND ...................-0.3V, (VL + 0.3V) Lead Temperature (soldering, 10sec) .............................+300°C
CSH, CSL to GND .......................................................-0.3V, +7V
VL Short Circuit to GND..............................................Momentary

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and 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 affect device reliability.

ELECTRICAL CHARACTERISTICS
(V+ = +15V, GND = PGND = 0V, IVL = IREF = 0A, TA = 0°C to +85°C, unless otherwise noted.)

PARAMETER CONDITIONS MIN TYP MAX UNITS

STEP-DOWN CONTROLLER
Input Supply Range 4.5 30 V
Nominal Adjustable Output
External resistor divider 1.6 6 V
Voltage Range
Feedback Voltage CSH - CSL = 40mV, SKIP = VL 1.576 1.6 1.624 V
Load Regulation 0mV < (CSH - CSL) < 80mV, SKIP = VL ±0.4 %
Line Regulation 6V < V+ < 30V 0.04 0.05 %/V
CSH - CSL, positive 80 100 120
Current-Limit Voltage mV
CSH - CSL, negative -40 -100 -160
SS Source Current 2.0 4.0 6.5 µA
SS Fault Sink Current 2.0 mA
INTERNAL REGULATOR AND REFERENCE
VL Output Voltage SHDN = 2V, 0mA <IVL < 25mA, 5.5V < V+ < 30V 4.75 5.25 V
VL Fault Lockout Voltage Rising edge, hysteresis = 15mV 3.8 4.0 V
VL/CSL Switchover Voltage Rising edge, hysteresis = 25mV 4.2 4.7 V
Reference Output Voltage No external load (Note 1) 2.463 2.505 2.537 V
Reference Fault Lockout Voltage Falling edge 1.8 2.3 V
Reference Load Regulation 0µA < IREF < 100µA 20 mV
CSL Shutdown Leakage Current SHDN = 0V, CSL = 6V, V+ = 0V or 30V, VL = 0V 0.1 1 µA
V+ Shutdown Current SHDN = 0V, V+ = 30V, CSL = 0V or 6V 1 5 µA
V+ Off-State Leakage Current FB = CSH = CSL = 6V, VL switched over to CSL 1 5 µA
Dropout Power Consumption V+ = 4V, CSL = 0V (Note 2) 6.6 10.5 mW
Quiescent Power Consumption CSH = CSL = 6V 6.4 8.5 mW

2 _______________________________________________________________________________________
 High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS (continued)

MAX798
(V+ = +15V, GND = PGND = 0V, IVL = IREF = 0A, TA = 0°C to +85°C, unless otherwise noted.)
PARAMETER CONDITIONS MIN TYP MAX UNITS
SYNC = REF 270 300 330
Oscillator Frequency kHz
SYNC = 0V or 5V 125 150 175
SYNC High Pulse Width 200 ns
SYNC Low Pulse Width 200 ns
SYNC Rise/Fall Time Guaranteed by design 200 ns
Oscillator Sync Range 195 340 kHz
SYNC = REF 89 91
Maximum Duty Cycle %
SYNC = 0V or 5V 93 96
SYNC VL - 0.5
Input High Voltage V
SHDN, SKIP 2.0
SYNC 0.8
Input Low Voltage V
SHDN, SKIP 0.5
SHDN, 0V or 30V 2
µA
SYNC, SKIP 1 µA
Input Current
CSH, CSL, CSH = CSL = 4V, device not shut down 50
FB, FB = 1.6V ±100 nA
DL Sink/Source Current DL forced to 2V 1 A
DH Sink/Source Current DH forced to 2V, BST - LX = 4.5V 1 A
DL On-Resistance High or low 7 Ω
DH On-Resistance High or low, BST - LX = 4.5V 7 Ω

_______________________________________________________________________________________ 3
 High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS
MAX798

(V+ = +15V, GND = PGND = 0V, IVL = IREF = 0A, TA = -40°C to +85°C, unless otherwise noted.) (Note 3)

PARAMETER CONDITIONS MIN TYP MAX UNITS

STEP-DOWN CONTROLLER
Input Supply Range 5.0 30 V
Nominal Adjustable Output
External resistor divider 1.6 6.0 V
Voltage Range
Feedback Voltage CSH - CSL = 40mV, SKIP = VL 1.560 1.6 1.640 V
Load Regulation 0mV < (CSH - CSL) < 80mV, SKIP = VL ±1 %
Line Regulation 6V < V+ < 30V 0.04 0.06 %/ V
CSH - CSL, positive 70 130
Current-Limit Voltage mV
CSH - CSL, negative -40 -100 -160
INTERNAL REGULATOR AND REFERENCE
VL Output Voltage SHDN = 2V, 0mA < IVL < 25mA, 5.5V < V+ < 30V 4.7 5.3 V
VL Fault Lockout Voltage Rising edge, hysteresis = 15mV 3.75 4.05 V
VL/CSL Switchover Voltage Rising edge, hysteresis = 25mV 4.15 4.75 V
Reference Output Voltage No external load (Note 1) 2.438 2.505 2.562 V
Reference Load Regulation 0µA < IREF < 100µA 30 mV
V+ Shutdown Current SHDN = 0V, V+ = 30V, CSL = 0V or 6V 1 10 µA
V+ Off-State Leakage Current FB = CSH = CSL = 6V, VL switched over to CSL 1 10 µA
Quiescent Power Consumption CSH = CSL = 6V 6.4 9.1 mW
OSCILLATOR AND INPUTS/OUTPUTS
SYNC = REF 250 300 350
Oscillator Frequency kHz
SYNC = 0V or 5V 110 150 190
SYNC High Pulse Width 250 ns
SYNC Low Pulse Width 250 ns
Oscillator Sync Range 210 320 kHz
SYNC = REF 88 91
Maximum Duty Cycle %
SYNC = 0V or 5V 92 96
DL On-Resistance High or low 7 Ω
DH On-Resistance High or low, BST - LX = 4.5V 7 Ω

Note 1: Since the reference uses VL as its supply, V+ line-regulation error is insignificant.
Note 2: At very low input voltages, quiescent supply current can increase due to excess PNP base current in the VL linear
regulator. This occurs only if V+ falls below the preset VL regulation point (5V nominal). The typical maximum quiescent
current in dropout will not exceed 16mA.
Note 3: All -40°C to +85°C specifications above are guaranteed by design.

4 _______________________________________________________________________________________
 High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
______________________________________________________________Pin Description

MAX798
PIN NAME FUNCTION

1 SS Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 1ms/nF.
Disables pulse-skipping mode when high. Connect to GND for normal use. Don’t leave SKIP unconnected.
2 SKIP With SKIP grounded, the device will automatically change from pulse-skipping operation to full PWM opera-
tion when the load current exceeds approximately 30% of maximum.
3 REF Reference Voltage Output. Bypass to GND with 0.33µF minimum.

4 GND Low-Noise Analog Ground and Feedback Reference Point

Oscillator Synchronization and Frequency Select. Tie to GND or VL for 150kHz operation; tie to REF for
5 SYNC 300kHz operation. A high-to-low transition begins a new cycle. Drive SYNC with 0V to 5V logic levels (see the
Electrical Characteristics table for VIH and VIL specifications). SYNC capture range is 195kHz to 340kHz.
Shutdown Control Input, active low. Logic threshold is set at approximately 1V (VTH of an internal N-channel
6 SHDN
MOSFET). Tie SHDN to V+ for automatic start-up.
7 FB Feedback Input. Regulates at FB = 1.6V. Connect FB to a resistor divider to set the output voltage.

8 CSH Current-Sense Input, high side. Current-limit level is 100mV referred to CSL.

9 CSL Current-Sense Input, low side

Battery Voltage Input (4.5V to 30V). Bypass V+ to PGND close to the IC with a 0.1µF capacitor. Connects to a
10 V+
linear regulator that powers VL.
5V Internal Linear-Regulator Output. VL is also the supply voltage rail for the chip. VL is switched to the out-
11 VL put voltage via CSL (VCSL > 4.5V) for automatic bootstrapping. Bypass to GND with 4.7µF. VL can supply
up to 5mA for external loads.
12 PGND Power Ground

13 DL Low-Side Gate-Drive Output. Normally drives the synchronous-rectifier MOSFET. Swings 0V to VL.

14 BST Boost Capacitor Connection for high-side gate drive (0.1µF)

15 LX Switching Node (inductor) Connection. Can swing 2V below ground without hazard.
High-Side Gate-Drive Output. Normally drives the main buck switch. DH is a floating driver output that swings
16 DH
from LX to BST, riding on the LX switching-node voltage.

_______________________________________________________________________________________ 5
 High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
MAX798

VIN, 4.5V TO 30V

R3
10Ω
2.2µF 4.7µF

C1 C2
0.33µF V+ SKIP VL
REF BST
DH Q1
0.1µF L1
ON SHDN 12µH 15mΩ
OFF LX VOUT, +2.5V @ 4.2A
C3
47pF C4
MAX798
DL Q2

PGND
C5
R4 R1
470pF
1M
CSH
0.01µF
SS CSL R2

FB

SYNC GND

Q1, Q2 = SILICONIX Si4410DY or IRF7413

C1, C2 = 10µF/30V SANYO OS-CON (30SA10)
C4 = 470µF/4V SPRAGUE 594D SERIES (594D477X0004R2T)
L1 = SUMIDA CDRH127 120
R1 = 6.49kΩ, 1%
R2 = 11.5kΩ, 1%
f= 150kHz

Figure 1. Standard Application Circuit

_______________Detailed Description mode that reduces losses due to MOSFET gate charge.
The step-down power-switching circuit consists of two
The MAX798 is a BiCMOS, switch-mode power-supply
N-channel MOSFETs, a rectifier, and an LC output filter.
controller designed primarily for buck-topology regula-
The output voltage is the average of the AC voltage at
tors in battery-powered applications where high accu-
the switching node, which is adjusted and regulated by
racy, high efficiency, and low quiescent supply current
changing the duty cycle of the MOSFET switches. The
are critical. The MAX798 also works well in other
gate-drive signal to the N -channel high-side MOSFET
topologies such as boost, inverting, and CUK due to
must exceed the battery voltage and is provided by a
the flexibility of its floating high-speed gate driver.
flying capacitor boost circuit that uses a 100nF capaci-
Light-load efficiency is enhanced by automatic idle-
tor connected between BST and LX.
mode operation—a variable-frequency pulse-skipping

6 _______________________________________________________________________________________
 High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
The MAX798 contains the following seven major circuit Bias Generator Blocks:

MAX798
blocks, which are shown in Figure 2. • +5V Linear Regulator
PWM Controller Blocks: • Automatic Bootstrap Switchover Circuit
• Multi-Input PWM Comparator • +2.505V Reference
• Current-Sense Circuit
• PWM Logic Block
• Gate-Driver Outputs

BATTERY VOLTAGE

V+
TO
CSL

+5V LINEAR OUT VL +5V AT 5mA

4.5V REGULATOR
SHDN

BST

SKIP PWM DH
LOGIC
LX MAIN
OUTPUT

DL
+2.505V
REF PGND
PWM
COMPARATOR
+2.505V CSH
AT 100µA REF
1X
1.6V CSL

GND

FB
ON/OFF
SHDN

SYNC
SS
MAX798

Figure 2. Functional Diagram

_______________________________________________________________________________________ 7
 High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
These internal IC blocks aren’t powered directly from Adjusting the Output Voltage
MAX798

the battery. Instead, a +5V linear regulator steps down The output voltage is set by an external resistor divider
the battery voltage to supply both the IC internal rail (VL between the output voltage and GND, with the midpoint
pin) as well as the gate drivers. The synchronous- connected to FB (Figure 5). The output voltage can be
switch gate driver is directly powered from +5V VL, adjusted from 1.6V to 6V, according to the formula in
while the high-side-switch gate driver is indirectly pow- Figure 5. Recommended R2 values range from 5k Ω to
ered from VL via an external diode-capacitor boost cir- 100kΩ. For best noise immunity, place R1 and R2 close
cuit. An automatic bootstrap circuit turns off the +5V to FB. For a 1.6V output, connect the output voltage
linear regulator and powers the IC from its output volt- directly to FB.
age if the output is above 4.5V. Remote sensing of the output voltage is easily achieved
PWM Controller Block by connecting the top of R1 and, if desired, the bottom
The heart of the current-mode PWM controller is a multi- of R2, to the remote sense point.
input open-loop comparator that sums three Bypassing and
signals: output voltage error signal with respect to the Compensation Components
reference voltage, current-sense signal, and slope The MAX798 is designed to deliver a more accurate
compensation ramp (Figure 3). The PWM controller is output voltage than the MAX797. A major source of the
a direct summing type, lacking a traditional error MAX797’s output error is the decrease in output voltage
amplifier and the phase shift associated with it. This with increasing load. This error is greatly reduced in the
direct-summing configuration approaches the ideal of MAX798 by increasing the gain of the voltage-sense
cycle-by-cycle control over the output voltage. signal relative to the current-sense signal. As a result of
For more comprehensive information on the MAX798 this increased gain, the MAX798 is slightly more noise
internal blocks, please refer to the MAX796/MAX797/ sensitive than the MAX797, and requires some small
MAX799 data sheet. compensation components as well. On the other hand,
output capacitor ESR requirements can be greatly
__________Applications Information relaxed compared to the MAX797, with the limiting fac-
Powering VL from a +5V Supply tor being the maximum total output voltage ripple that
If the circuit’s output voltage is greater than the VL/CSL the application can tolerate.
switchover voltage, the IC automatically bootstraps and To control noise problems, place the bypass capacitors
runs off its output. Running from the high-efficiency out- on REF, VL, and V+ as close as possible to the IC, and
put rather than the VL linear regulator is particularly use a 10Ω series resistor (R3, Figure 1) on V+ to form a
desirable if the input voltage is high. If the output is small lowpass filter. Feed-forward components (R4, C3,
not greater than the VL/CSL switchover threshold, and C5) are chosen for stable switching at 150kHz with
efficiency can be improved by powering VL from the components shown. For 300kHz switching and a
another efficient system supply (Figure 4). 4.7µH inductor, use R4 = 470k Ω and C5 = 220pF.
VL can be connected directly to a +5V supply, provid- Keep the components that connect to FB (R4, C5, R1,
ed its maximum excursions are within a 4.5V to 6V R2) close to the IC’s FB pin.
range and the main output voltage’s maximum is less Design Procedure
than 4.2V. If the circuit’s output voltage is greater than With the exception of the items previously mentioned,
4.2V, the IC can activate its internal bootstrap switch follow the design procedure for the MAX797. The
and connect the circuit output to VL. MAX796/MAX797/MAX799 data sheet contains all nec-
essary information on component values, component
selection, layout, and additional applications.

8 _______________________________________________________________________________________
 High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power

MAX798
CSH

1X
CSL
1.6V
FROM
FEEDBACK
MAIN PWM DIVIDER
COMPARATOR

BST
R
Q LEVEL
DH
S SHIFT

LX

SLOPE COMP OSC

30mV
SKIP

VL

4µA CURRENT
SHOOT-
LIMIT
THROUGH
CONTROL

SS 24R

2.5V 1R MAX798
SHDN N

SYNCHRONOUS
RECTIFIER CONTROL
VL
R
Q
LEVEL DL
–100mV
S SHIFT

PGND

Figure 3. PWM Controller Detailed Block Diagram

_______________________________________________________________________________________ 9
 High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
MAX798

V+
5V ±10% VBATT REMOTE
SENSE
DH LINES MAIN
OUTPUT
0.1µF

V+ VL
MAX798
DL

CSH R1
MAX798
CSL
DH
FB
VOUT GND
R2
DL (VOUT (MAX) < 4.2V)

R1
(
VOUT = 1.6V 1 + –––
R2
)

Figure 4. Powering VL from a Separate +5V Supply Figure 5. Adjusting the Output Voltage

10 ______________________________________________________________________________________
 High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
___________________Chip Information

MAX798
TRANSISTOR COUNT: 1008

________________________________________________________Package Information
INCHES MILLIMETERS
DIM
MIN MAX MIN MAX
A 0.053 0.069 1.35 1.75
D A1 0.004 0.010 0.10 0.25
B 0.014 0.019 0.35 0.49
0°-8° C 0.007 0.010 0.19 0.25
A
E 0.150 0.157 3.80 4.00
0.101mm
0.004in.
e 0.050 1.27
e H 0.228 0.244 5.80 6.20
B A1 C L L 0.016 0.050 0.40 1.27

INCHES MILLIMETERS
Narrow SO DIM PINS
MIN MAX MIN MAX

E H
SMALL-OUTLINE D 8 0.189 0.197 4.80 5.00
D 14 0.337 0.344 8.55 8.75
PACKAGE
D 16 0.386 0.394 9.80 10.00
(0.150 in.) 21-0041A

______________________________________________________________________________________ 11
 High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
NOTES
MAX798

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. N o circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600

© 1996 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.