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_______________General Description

The MAX797H high-performance, step-down DC-DC

converter provides main CPU power in battery-powered

systems. A 40V rating on the power stage’s input allows

operation with high-cell-count batteries and a wide

range of AC adaptors. This buck controller achieves

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 tran-

sients 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 MAX797H has a logic-controlled and synchronizable

fixed-frequency, pulse-width-modulating (PWM) operating

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

light loads, or forces fixed-frequency mode for lowest noise

at all loads. The MAX797H is pin compatible with the popu-

lar MAX797, but has a higher input voltage range.

The MAX797H comes in a 16-pin narrow SO package.

________________________Applications

Notebook and Subnotebook Computers

Industrial Controls

____________________________Features

♦ 96% Efficiency

♦ Up to 40V Power Input

♦ 2.5V to 6V Adjustable Output

♦ Preset 3.3V and 5V Outputs (at up to 10A)

♦ 5V Linear-Regulator Output

♦ Precision 2.505V Reference Output

♦ Automatic Bootstrap Circuit

♦ 150kHz/300kHz Fixed-Frequency PWM Operation

♦ Programmable Soft-Start

♦ 375µA Quiescent Current (V

= 12V, V

OUT

= 5V)

♦ 1µA Shutdown Current

MAX797H

High-Voltage, Step-Down Controller with

Synchronous Rectifier for CPU Power

________________________________________________________________

Maxim Integrated Products

Idle Mode is a trademark of Maxim Integrated Products.

19-1239; Rev 0; 7/97

__________________Pin Configuration

BST

DLGND

REF

SKIP

TOP VIEW

MAX797H

PGND

V+

CSL

CSH

SHDN

SYNC

__________Typical Operating Circuit

MAX797H

SHDN

+3.3V

OUTPUT

+4V TO +40V

POWER INPUT

BST

PGND

CSH

CSL

REF

SYNC

GND

SKIP FB

V+

+4.5V TO +30V

SUPPLY

INPUT

PIN-PACKAGETEMP. RANGEPART

†

16 Narrow SO-40°C to +85°CMAX797HESE

For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800

For small orders, phone 408-737-7600 ext. 3468.

______________Ordering Information

†

Summary of content (8 pages)

PAGE 1
9-1239; Rev 0; 7/97 High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power The MAX797H comes in a 16-pin narrow SO package. ________________________Applications Notebook and Subnotebook Computers ____________________________Features ♦ 96% Efficiency ♦ Up to 40V Power Input ♦ 2.5V to 6V Adjustable Output ♦ ♦ ♦ ♦ Preset 3.3V and 5V Outputs (at up to 10A) 5V Linear-Regulator Output Precision 2.
PAGE 2
MAX797H High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power ABSOLUTE MAXIMUM RATINGS V+ to GND ................................................................-0.3V to 36V GND to PGND........................................................................±2V VL to GND...................................................................-0.3V to 7V BST to GND ..............................................................-0.3V to 46V DH to LX .............................................
PAGE 3
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power (V+ = 15V, GND = PGND = 0V, IVL = IREF = 0A, TA = 0°C to +85°C, SYNC = 0V, unless otherwise noted.) MIN TYP MAX UNITS Reference Output Voltage PARAMETER No external load (Note 1) CONDITIONS 2.45 2.505 2.55 V Reference Fault Lockout Voltage Falling edge 1.
PAGE 4
MAX797H High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power ELECTRICAL CHARACTERISTICS (continued) (V+ = 15V, GND = PGND = 0V, IVL = IREF = 0A, TA = -40 to +85°C, SYNC = 0V, unless otherwise noted.) (Note 3) PARAMETER CONDITIONS MIN TYP MAX UNITS +3.3V and +5V STEP-DOWN CONTROLLERS Input Supply Range V+ 5.0 30 High-side MOSFET drain 40 V 5V Output Voltage (CSL) 0mV < (CSH - CSL) < 80mV, FB = VL, 6V < power input < 40V, includes line and load regulation (Note 4) 4.
PAGE 5
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power PIN NAME 1 SS 2 SKIP Disables pulse-skipping mode when high. Connect SKIP to GND for normal use. Do not leave unconnected. With SKIP grounded, the device automatically changes from pulse-skipping operation to full PWM operation when the load current exceeds approximately 30% of maximum. 3 REF Reference Voltage Output. Bypass REF to GND with 0.33µF minimum.
PAGE 6
MAX797H High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power _______________Detailed Description The MAX797H is functionally identical to the MAX797. The only difference between the two devices is in the BST pin’s absolute maximum rating. The MAX797H’s rating is 46V; the MAX797’s rating is 36V. The higher rating allows the MAX797H to use a power input up to 40V, provided that the V+ pin is powered by a separate supply between 4.5V and 30V.
PAGE 7
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power V+ MAX797H +5V REGULATED SUPPLY +4V TO +40V POWER INPUT VL SHDN MAX797H DH BST SS LX REF DL +3.3V OUTPUT PGND CSH SYNC GND SKIP CSL FB Figure 1. Powering V+ and VL from a Regulated +5V supply Similarly, at 150kHz, the minimum duty factor is 0.425µs / 6.67µs = 0.064. This means that duty factor is not an issue except at the maximum input voltage and minimum output voltage.
PAGE 8
MAX797H High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power OUT +8V TO +40V POWER INPUT IN ADJ 100k R1 +7V TO +40V INPUT Q1 300k R2 V+ V+ VL VL SHDN SHDN MAX797H MAX797H DH Figure 2. Powering V+ and VL with a +5V Linear Regulator DH Figure 3. Dividing the Power Input to Supply V+ +20V TO +40V POWER INPUT UP TO +40V POWER INPUT 12V R1 R1 V+ V+ VL VL SHDN SHDN MAX797H MAX797H DH Figure 4. Powering V+ with a Zener Shunt Regulator DH Figure 5.