Rainbow Electronics MAX799 User Manual

_______________General Description

The MAX796/MAX797/MAX799 high-performance, step­down DC-DC converters with single or dual outputs
provide main CPU power in battery-powered systems.
These buck controllers achieve 96% efficiency by using
synchronous rectification and Maxim’s proprietary Idle
Mode™ control scheme to extend battery life at full-load
(up to 10A) and no-load outputs. Excellent dynamic
response corrects output transients caused by the latest
dynamic-clock CPUs within five 300kHz clock cycles.
Unique bootstrap circuitry drives inexpensive N-channel
MOSFETs, reducing system cost and eliminating the
crowbar switching currents found in some PMOS/NMOS
switch designs.

The MAX796/MAX799 are specially equipped with a sec­ondary feedback input (SECFB) for transformer-based
dual-output applications. This secondary feedback path
improves cross-regulation of positive (MAX796) or nega­tive (MAX799) auxiliary outputs.

The MAX797 has a logic-controlled and synchronizable
fixed-frequency pulse-width-modulating (PWM) operating
mode, which reduces noise and RF interference in sensi­tive mobile-communications and pen-entry applications.
The SKIP override input allows automatic switchover to
idle-mode operation (for high-efficiency pulse skipping) at
light loads, or forces fixed-frequency mode for lowest noise
at all loads.

The MAX796/MAX797/MAX799 are all available in 16­pin DIP and narrow SO packages. See the table below
to compare these three converters.

________________________Applications

Notebook and Subnotebook Computers
PDAs and Mobile Communicators
Cellular Phones

____________________________Features

♦ 96% Efficiency
♦ 4.5V to 30V Input Range
♦ 2.5V to 6V Adjustable Output
♦ Preset 3.3V and 5V Outputs (at up to 10A)
♦ Multiple Regulated Outputs
♦ +5V Linear-Regulator Output
♦ Precision 2.505V Reference Output
♦ Automatic Bootstrap Circuit
♦ 150kHz/300kHz Fixed-Frequency PWM Operation
♦ Programmable Soft-Start
♦ 375µA Typ Quiescent Current (VIN= 12V, V

OUT

= 5V)

♦ 1µA Typ Shutdown Current

MAX796/MAX797/MAX799

Step-Down Controllers with

Synchronous Rectifier for CPU Power

________________________________________________________________

Maxim Integrated Products

1

PART

MAX799

MAIN OUTPUT SPECIAL FEATURE

3.3V/5V or adj.

Regulates negative secondary
voltage (such as -5V)

MAX797 3.3V/5V or adj. Logic-controlled low-noise mode

MAX796 3.3V/5V or adj.

Regulates positive secondary
voltage (such as +12V)

Idle Mode is a trademark of Maxim Integrated Products.

†

U.S. and foreign patents pending.

19-0221; Rev 3a; 11/97

EVALUATION KIT MANUALS

FOLLOW DATA SHEET

__________________Pin Configuration

16
15
14
13
12
11
10

9

1
2
3
4
5
6
7
8

DH
LX
BST
DL

GND

REF

(SECFB) SKIP

SS

TOP VIEW

MAX796
MAX797
MAX799

PGND
VL
V+
CSL

( ) ARE FOR MAX796/ MAX799.

CSH

FB

SHDN

SYNC

DIP/SO

Dice*

16 Narrow SO

16 Plastic DIP

PIN-PACKAGETEMP. RANGE

0°C to +70°C
0°C to +70°C
0°C to +70°CMAX796C/D

MAX796CSE

MAX796CPE

PART

†

16 CERDIP

16 Narrow SO

16 Plastic DIP-40°C to +85°C

-40°C to +85°C

-55°C to +125°CMAX796MJE

MAX796ESE

MAX796EPE

Ordering Information continued at end of data sheet.

*Contact factory for dice specifications.

______________Ordering Information

For free samples & the latest literature: http://www.maxim-ic.com, or phone [phone].
For small orders, phone [phone] ext. 3468.

MAX796/MAX797/MAX799

Step-Down Controllers with
Synchronous Rectifier for CPU Power

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(V+ = 15V, GND = PGND = 0V, IVL= I

REF

= 0A, TA= 0°C to +70°C for MAX79_C, TA= 0°C to +85°C for MAX79_E,

T

A

= -55°C to +125°C for MAX79_M, unless otherwise noted.)

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.

V+ to GND.................................................................-0.3V, +36V

GND to PGND........................................................................±2V

VL to GND ...................................................................-0.3V, +7V

BST to GND...............................................................-0.3V, +36V

DH to LX...........................................................-0.3V, BST + 0.3V

LX to BST.....................................................................-7V, +0.3V

SHDN

to GND............................................................-0.3V, +36V

SYNC, SS, REF, FB, SECFB, SKIP

, DL to GND..-0.3V, VL + 0.3V

CSH, CSL to GND .......................................................-0.3V, +7V

VL Short Circuit to GND..............................................Momentary

REF Short Circuit to GND...........................................Continuous

VL Output Current...............................................................50mA

Continuous Power Dissipation (T

A

= +70°C)

SO (derate 8.70mW/°C above +70°C)........................696mW

Plastic DIP (derate 10.53mW/°C above +70°C) .........842mW

CERDIP (derate 10.00mW/°C above +70°C)..............800mW

Operating Temperature Ranges

MAX79_C_ _ ......................................................0°C to +70°C

MAX79_E_ _....................................................-40°C to +85°C

MAX79_MJE .................................................-55°C to +125°C

Storage Temperature Range.............................-65°C to +160°C

Lead Temperature (soldering, 10sec).............................+300°C

Rising edge, hysteresis = 25mV

Rising edge, hysteresis = 15mV

SHDN = 2V, 0mA < IVL< 25mA, 5.5V < V+ < 30V

Falling edge, hysteresis = 20mV (MAX799)

CSH-CSL, negative

CSH-CSL, positive

Falling edge, hysteresis = 15mV (MAX796)

6V < V+ < 30V

25mV < (CSH-CSL) < 80mV

0mV < (CSH-CSL) < 80mV, FB = VL, 6V < V+ < 30V,
includes line and load regulation

External resistor divider
(CSH-CSL) = 0V

0mV < (CSH-CSL) < 80mV

CONDITIONS

V4.2 4.7VL/CSL Switchover Voltage

V3.8 4.1VL Fault Lockout Voltage

V4.7 5.3VL Output Voltage

-0.05 0 0.05

V

2.45 2.505 2.55

SECFB Regulation Setpoint

mA2.0SS Fault Sink Current

µA2.5 4.0 6.5SS Source Current

5.0 30

V

4.5 30

Input Supply Range

-50 -100 -160

mV

80 100 120

Current-Limit Voltage

%/V0.04 0.06Line Regulation

1.5

V4.85 5.10 5.255V Output Voltage (CSL)

VREF 6

Nominal Adjustable Output
Voltage Range

V2.43 2.505 2.57Feedback Voltage

%

2.5

Load Regulation

UNITSMIN TYP MAXPARAMETER

MAX79_C
MAX79_E/M

0mV < (CSH-CSL) < 80mV, FB = 0V, 4.5V < V+ < 30V,
includes line and load regulation

V3.20 3.35 3.463.3V Output Voltage (CSL)

+3.3V AND +5V STEP-DOWN CONTROLLERS

FLYBACK/PWM CONTROLLER

INTERNAL REGULATOR AND REFERENCE

MAX796/MAX797/MAX799

Step-Down Controllers with

Synchronous Rectifier for CPU Power

_______________________________________________________________________________________ 3

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 may 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). See the Quiescent Supply Current
vs. Supply Voltage graph in the

Typical Operating Characteristics

.

ELECTRICAL CHARACTERISTICS (continued)

(V+ = 15V, GND = PGND = 0V, IVL= I

REF

= 0A, TA= 0°C to +70°C for MAX79_C, TA= 0°C to +85°C for MAX79_E,

T

A

= -55°C to +125°C for MAX79_M, unless otherwise noted.)

SECFB, 0V or 4V

SHDN, 0V or 30V

SHDN, SKIP

SYNC

SYNC = 0V or 5V

No external load (Note 1)

SYNC = REF

Guaranteed by design

CSH = CSL = 6V

V+ = 4V, CSL = 0V (Note 2)

SYNC = 0V or 5V

SHDN = 0V, V+ = 30V,
CSL = 0V or 6V

Falling edge
0µA < I

REF

< 100µA

SYNC = REF

SHDN = 0V, CSL = 6V, V+ = 0V or 30V, VL = 0V

CONDITIONS

0.1

MAX79_C

µA

2.0

MAX79_E/M

Input Current

2.0

V

VL - 0.5

Input High Voltage

%

93 96

89 91

Maximum Duty Cycle

kHz190 340Oscillator Sync Range

ns200SYNC Rise/Fall Time

ns200SYNC Low Pulse Width

ns200SYNC High Pulse Width

125 150 175

kHz

270 300 330

Oscillator Frequency

2.45 2.55

V

2.46 2.505 2.54

Reference Output Voltage

mW4.8 6.6Quiescent Power Consumption

mW4 8Dropout Power Consumption

1 5

µA

1 3

V+ Shutdown Current

V1.8 2.3Reference Fault Lockout Voltage

mV50Reference Load Regulation

µA0.1 1CSL Shutdown Leakage Current

UNITSMIN TYP MAXPARAMETER

MAX79_C
MAX79_E/M
MAX79_C
MAX79_E/M

FB = CSH = CSL = 6V,
VL switched over to CSL

1 5

µA

1 3

V+ Off-State Leakage Current

DL forced to 2V

FB, FB = REF

CSH, CSL, CSH = CSL = 6V, device not shut down

SYNC, SKIP

A1DL Sink/Source Current

±100

50

1.0

SHDN, SKIP

SYNC

0.5

V

0.8

Input Low Voltage

DH forced to 2V, BST-LX = 4.5V A1DH Sink/Source Current

High or low, BST-LX = 4.5V

High or low

Ω7DH On-Resistance

Ω7DL On-Resistance

OSCILLATOR AND INPUTS/OUTPUTS

nA

MAX796/MAX797/MAX799

Step-Down Controllers with
Synchronous Rectifier for CPU Power

4 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

(V+ = 15V, GND = PGND = 0V, IVL= I

REF

= 0A, TA= -40°C to +85°C for MAX79_E, unless otherwise noted.) (Note 3)

Note 3: All -40°C to +85°C specifications above are guaranteed by design.

External resistor divider

0mV < (CSH - CSL) < 80mV, FB = VL, 4.5V < V+ < 30V,
includes line and load regulation

0mV < (CSH - CSL) < 80mV, FB = VL, 6V < V+ < 30V,
includes line and load regulation

CONDITIONS

VREF 6.0

Nominal Adjustable Output
Voltage Range

V3.10 3.35 3.563.3V Output Voltage (CSL)

V5.0 30Input Supply Range
V4.70 5.10 5.405V Output Voltage (CSL)

UNITSMIN TYP MAXPARAMETER

CSH - CSL, negative

(CSH-CSL) = 0V
6V < V+ < 30V
CSH - CSL, positive

-40 -100 -160

Current-Limit Voltage

V2.40 2.60Feedback Voltage

%/V0.04 0.06Line Regulation

mV

70 130

FB = CSH = CSL = 6V, VL switched over to CSL

SHDN = 0V, V+ = 30V, CSL = 0V or 6V

Rising edge, hysteresis = 25mV
No external load (Note 1)
0µA < I

REF

< 100µA

µA1 10V+ Off-State Leakage Current

µA1 10V+ Shutdown Current

V

Rising edge, hysteresis = 15mV

SHDN = 2V, 0mA < IVL< 25mA, 5.5V < V+ < 30V

4.2 4.7VL/CSL Switchover Voltage
V2.43 2.505 2.57Reference Output Voltage

Falling edge, hysteresis = 15mV (MAX796)
Falling edge, hysteresis = 20mV (MAX799)

mV50Reference Load Regulation

V3.75 4.05VL Fault Lockout Voltage

V4.7 5.3VL Output Voltage

2.40 2.60
V

-0.08 0.08

SECFB Regulation Setpoint

SYNC = REF

SYNC = 0V or 5V

89 91

kHz210 320Oscillator Sync Range

kHz

SYNC = REF

120 150 180

Oscillator Frequency

ns250SYNC High Pulse Width
ns250SYNC Low Pulse Width

250 300 350

mW4.8 8.4Quiescent Power Consumption

High or low, BST - LX = 4.5V

High or low

SYNC = 0V or 5V

Ω7DH On-Resistance

Ω7DL On-Resistance

%

93 96

Maximum Duty Cycle

+3.3V and +5V STEP-DOWN CONTROLLERS

FLYBACK/PWM CONTROLLER

INTERNAL REGULATOR AND REFERENCE

OSCILLATOR AND INPUTS/OUTPUTS

MAX796/MAX797/MAX799

Step-Down Controllers with

Synchronous Rectifier for CPU Power

_______________________________________________________________________________________ 5

SHDN

DH

+12V

OUTPUT

+5V

OUTPUT

INPUT

6V TO 30V

BST

LX

DL

PGND

CSH

CSL

SS

REF

SYNC

GND

V+

VL

FB

SECFB

MAX796

__________________________________________________Typical Operating Circuits

MAX797

SHDN

DH

+3.3V

OUTPUT

INPUT

4.5V TO 30V

BST

LX

DL

PGND

CSH

CSL

SS

REF

SYNC

GND

SKIP FB

V+ VL

__________________________________________Typical Operating Characteristics

(TA = +25°C, unless otherwise noted.)

100

50

0.001 10.10.01 10

EFFICIENCY vs.

LOAD CURRENT, 5V/3A CIRCUIT

60

MAX796-01

LOAD CURRENT (A)

EFFICIENCY (%)

70

80

90

STANDARD MAX797 5V/3A
CIRCUIT, FIGURE 1
f = 300kHz

VIN = 6V

VIN = 30V

100

50

0.001 10.10.01 10

EFFICIENCY vs.

LOAD CURRENT, 3.3V/3A CIRCUIT

60

MAX796-02

LOAD CURRENT (A)

EFFICIENCY (%)

70

80

90

STANDARD MAX797 3.3V/3A
CIRCUIT, FIGURE 1
f = 300kHz

VIN = 12V

VIN = 30V

VIN = 5V

100

40

50

60

70

80

90

0.1 1 10

EFFICIENCY vs.

LOAD CURRENT, 3.3V/10A CIRCUIT

MAX796-03

LOAD CURRENT (A)

EFFICIENCY (%)

SKIP = LOW

SKIP = HIGH

STANDARD MAX797 3.3V/10A
CIRCUIT, FIGURE 1
f = 300kHz
V

IN

= 5V

MAX796/MAX797/MAX799

Step-Down Controllers with
Synchronous Rectifier for CPU Power

6 _______________________________________________________________________________________

MAX799

SHDN

DH

–5V
OUTPUT

+5V
OUTPUT

INPUT 6V TO 30V

BST

LX

DL

PGND

CSH

CSL

SS

REF

FROM

REF

SYNC

GND

V+

VL

FB

SECFB

_____________________________________Typical Operating Circuits (continued)

MAX796/MAX797/MAX799

Step-Down Controllers with

Synchronous Rectifier for CPU Power

_______________________________________________________________________________________

7

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

0

200µ

400µ

600µ

800µ

14m

15m

16m

0 4 8 12 16 20 24 28 32

QUIESCENT SUPPLY CURRENT

vs. SUPPLY VOLTAGE,

5V/3A CIRCUIT IN IDLE MODE

MAX796-04

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (A)

STANDARD MAX797 APPLICATION
CONFIGURED FOR 5V
SKIP = LOW
SYNC = REF

0

0.2

0.4

0.6

0.8

1.2

1.0

1.4

1.6

0 4 8 12 16 20 24 28 32

SHUTDOWN SUPPLY CURRENT

vs. SUPPLY VOLTAGE

MAX796-07

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (µA)

DEVICE CURRENT ONLY
SHDN = LOW

0

200

400

600

800

1200

1000

1400

0 4 8 12 16 20 24 28 32

MAX796-05

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (µA)

QUIESCENT SUPPLY CURRENT

vs. SUPPLY VOLTAGE,

3.3V/3A CIRCUIT IN IDLE MODE

STANDARD MAX797 3.3V/3A
CIRCUIT, FIGURE 1
SKIP = LOW
SYNC = REF

SWITCHING

NOT SWITCHING
(FB FORCED TO 3.5V)

0

10

20

30

0 4 8 12 16 20 24 28 32

MAX796-06

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (mA)

QUIESCENT SUPPLY CURRENT vs.

SUPPLY VOLTAGE, LOW-NOISE MODE

f = 150kHz

f = 300kHz

STANDARD MAX797 3.3V/3A
CIRCUIT, FIGURE 1
SKIP = HIGH

0

0.01 1 100.1

DROPOUT VOLTAGE vs.

LOAD CURRENT

200
100

300

400

500

600

700

800

MAX796-08

LOAD CURRENT (A)

V

IN

- V

OUT

(mV)

STANDARD MAX797 APPLICATION
CONFIGURED FOR 5V
V

OUT

> 4.8V

f = 150kHz

f = 300kHz

0

1 100 100010

REF LOAD-REGULATION ERROR

vs. LOAD CURRENT

5

10

15

20

MAX796-09

REF LOAD CURRENT (µA)

LOAD REGULATION ∆V (mV)

0

200

100

300

400

500

0 20 40 60 80

MAX796-10

VL LOAD CURRENT (mA)

LOAD REGULATION ∆V (mV)

VL LOAD-REGULATION ERROR

vs. LOAD CURRENT

0

50

100

150

200

250

300

350

400

450

0 4 8 12 16 20 24 28 32

MAX796-11

SUPPLY VOLTAGE (V)

MAXIMUM SECONDARY CURRENT (mA)

MAX796

MAXIMUM SECONDARY CURRENT

vs. SUPPLY VOLTAGE, 5V/15V CIRCUIT

I

OUT

(MAIN) = 0A

I

OUT

(MAIN) = 3A

CIRCUIT OF FIGURE 11
TRANSFORMER = TTI5870
V

SEC

> 12.75V

1000

0.1
100µ 10m 1

SWITCHING FREQUENCY vs.

LOAD CURRENT

10

LOAD CURRENT (A)

SWITCHING FREQUENCY (kHz)

100

1m 100m

SYNC = REF (300kHz)
SKIP = LOW

+5V, VIN = 7.5V

1

+5V, VIN = 30V

+3.3V, VIN = 7.5V

I

LOAD

= 100mA, VIN = 10V,

CIRCUIT OF FIGURE 1

IDLE-MODE WAVEFORMS

+5V OUTPUT
50mV/div

2V/div

200µs/div

I

LOAD

= 1A, VIN = 16V,

CIRCUIT OF FIGURE 1

PULSE-WIDTH-MODULATION MODE WAVEFORMS

LX VOLTAGE
10V/div

+5V OUTPUT
VOLTAGE
50mV/div

500ns/div

VIN = 15V, CIRCUIT OF FIGURE 1

+5V LOAD-TRANSIENT RESPONSE

+5V OUTPUT
50mV/div

3A
0A

LOAD CURRENT

200µs/div

MAX796/MAX797/MAX799

Step-Down Controllers with
Synchronous Rectifier for CPU Power

8 _______________________________________________________________________________________

0

150

300

450

600

900

750

1050

0 3 6 9 12 15 18 21 24

MAX796

MAXIMUM SECONDARY CURRENT vs.

SUPPLY VOLTAGE, 3.3V/5V CIRCUIT

MAX796-12

SUPPLY VOLTAGE (V)

MAXIMUM SECONDARY CURRENT (mA)

I

OUT

(MAIN) = 2A

I

OUT

(MAIN) = 0A

CIRCUIT OF FIGURE 12
TRANSFORMER = TDK 1.5:1
V

SEC

≥ 4.8V

0

100

200

300

400

600
500

700

800

0 4 8 12 16 20 24 28 32

MAX799

MAXIMUM SECONDARY CURRENT

vs. SUPPLY VOLTAGE, ±5V CIRCUIT

MAX796-13

SUPPLY VOLTAGE (V)

MAXIMUM SECONDARY CURRENT (mA)

CIRCUIT OF FIGURE 13
TRANSFORMER = TTI5926
V

SEC

≤ -5.1V

I

OUT

(MAIN) = 0A

I

OUT

(MAIN) = 1A

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

MAX796/MAX797/MAX799

Step-Down Controllers with

Synchronous Rectifier for CPU Power

_______________________________________________________________________________________ 9

______________________________________________________________Pin Description

Current-Sense input, High side. Current-limit level is 100mV referred to CSL.CSH8
Current-Sense input, Low side. Also serves as the feedback input in fixed-output modes.CSL9
Battery voltage input (4.5V to 30V). Bypass V+ to PGND close to the IC with a 0.1µF capacitor. Connects to a

linear regulator that powers VL.

V+10

5V Internal linear-regulator output. VL is also the supply voltage rail for the chip. VL is switched to the output
voltage via CSL (V

CSL

> 4.5V) for automatic bootstrapping. Bypass to GND with 4.7µF. VL can

supply up to 5mA for external loads.

VL11

Power Ground.PGND12

Low-noise analog Ground and feedback reference point.GND4
Oscillator Synchronization and frequency select. Tie to GND or VL for 150kHz operation; tie to REF for

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 VIHand VILspecifications). SYNC capture range is 190kHz to 340kHz

guaranteed.

SYNC5

Shutdown control input, active low. Logic threshold is set at approximately 1V (VTHof an internal N-channel
MOSFET). Tie SHDN to V+ for automatic start-up.

SHDN6

Feedback input. Regulates at FB = REF (approximately 2.505V) in adjustable mode. FB is a Dual-Mode

TM

input that also selects the fixed output voltage settings as follows:

• Connect to GND for 3.3V operation.

• Connect to VL for 5V operation.

• Connect FB to a resistor divider for adjustable mode. FB can be driven with +5V rail-to-rail logic in order to

change the output voltage under system control.

FB7

Reference voltage output. Bypass to GND with 0.33µF minimum.REF3

PIN

Secondary winding Feedback input. Normally connected to a resistor divider from an auxiliary output.

Don’t leave SECFB unconnected.

• MAX796: SECFB regulates at VSECFB = 2.505V. Tie to VL if not used.

• MAX799: SECFB regulates at VSECFB = 0V. Tie to a negative voltage through a high-value current-limit-

ing resistor (I

MAX

= 100µA) if not used.

SECFB

(MAX796/

MAX799)

2

Soft-Start timing capacitor connection. Ramp time to full current limit is approximately 1ms/nF.SS1

FUNCTIONNAME

Low-side gate-drive output. Normally drives the synchronous-rectifier MOSFET. Swings 0V to VL.DL13
Boost capacitor connection for high-side gate drive (0.1µF). BST14
Switching node (inductor) connection. Can swing 2V below ground without hazard.LX15
High-side gate-drive output. Normally drives the main buck switch. DH is a floating driver output that swings

from LX to BST, riding on the LX switching-node voltage.

DH16

Dual Mode is a trademark of Maxim Integrated Products.

Disables pulse-skipping mode when high. Connect to GND for normal use. Don’t leave SKIP unconnected.
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. (See Table 3.)

SKIP

(MAX797)

MAX796/MAX797/MAX799

Step-Down Controllers with
Synchronous Rectifier for CPU Power

10 ______________________________________________________________________________________

______Standard Application Circuit

It is easy to adapt the basic MAX797 single-output 3.3V
buck converter (Figure 1) to meet a wide range of
applications with inputs up to 28V (limited by choice of
external MOSFET). Simply substitute the appropriate
components from Table 1. These circuits represent a
good set of tradeoffs between cost, size, and efficiency
while staying within the worst-case specification limits
for stress-related parameters such as capacitor ripple
current. Each of these circuits is rated for a continuous
load current at TA= +85°C, as shown. The 1A, 2A and
10A applications can withstand a continuous output
short-circuit to ground. The 3A and 5A applications can
withstand a short circuit of many seconds duration, but
the synchronous-rectifier MOSFET overheats, exceed­ing the manufacturer’s ratings for junction temperature
by 50°C or more.

If the 3A or 5A circuit must be guaranteed to withstand
a continuous output short circuit indefinitely, see the
section

MOSFET Switches

under

Selecting Other

Components

. Don’t change the frequency of these cir­cuits without first recalculating component values (par­ticularly inductance value at maximum battery voltage).

_______________Detailed Description

The MAX796 is a BiCMOS, switch-mode power-supply
controller designed primarily for buck-topology regula­tors in battery-powered applications where high effi­ciency and low quiescent supply current are critical.
The MAX796 also works well in other topologies such
as boost, inverting, and CLK due to the flexibility of its
floating high-speed gate driver. Light-load efficiency is
enhanced by automatic idle-mode operation—a vari­able-frequency pulse-skipping mode that reduces

MAX797

CSL

CSH

VL

SYNC

FB

V+

10 11

57

14

Q1

Q2

16

15

13

D2
CMPSH-3

J1
150kHz/300kHz
JUMPER

NOTE: KEEP CURRENT-SENSE
LINES SHORT AND CLOSE
TOGETHER. SEE FIG. 10

D1

12
8
9

REF

3

GND

4

+5V AT
5mA

+3.3V
OUTPUT

GND
OUT

BST

DH

LX

DL

2

1

LOW-NOISE

CONTROL

PGND

SKIP

SS

6

ON/OFF

CONTROL

SHDN

INPUT

REF OUTPUT
+2.505V AT 100µA

C5

0.33µF

C4

4.7µF

C7

0.1µF

C6

0.01µF

(OPTIONAL)

C1

C2

C3

0.1µF
R1

L1

Figure 1. Standard 3.3V Application Circuit