High-Efficiency PWM, Step-Down

P-Channel DC-DC Controller

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5

PIN

NAME

FUNCTION

1

LBI

Input to the internal low-battery comparator. Tie to V+ or GND if not used.

2

SS

Soft-start limits start-up surge currents. On power-up, it charges the soft-start capacitor, slowly raising the

peak current limit to the level set by the sense resistor.

3

REF

2V reference output that can source 100µA for external loads. Bypass with 0.22µF. The reference is

disabled in shutdown mode.

4

SHDN

to 20µA.

5

FB

Feedback input for adjustable-output operation. Connect to GND for fixed +5V output. Use a resistor

divider network to adjust the output voltage. See the section

Setting the Output Voltage.

6

CC

Compensation capacitor. AC compensation input for the error amplifier. Connect a capacitor between CC

and GND for fixed +5V output operation. See

Compensation Capacitor section.

7

AV+

Quiet supply voltage for sensitive analog circuitry. A bypass capacitor is not required for AV+.

8

OUT

Output voltage sense input. Connects to internal resistor divider. Leave unconnected for adjustable output.

Bypass to AGND with a 0.1µF capacitor close to the IC.

9

CS

10

AGND

Quiet analog ground

11

EXT

Power MOSFET gate drive output that swings between V+ and GND. EXT is not protected against short

circuits to V+ or AGND.

12

V+

High-current supply voltage for the output driver

13

GND

High-current ground return for the output driver

14

LBO

Low-battery output is an open-drain output that goes low when LBI is less than 2V. Connect to V+ through

a pull-up resistor. Leave floating if not used. LBO is disabled in shutdown mode.

______________________________________________________________Pin Description

____________________Getting Starting

Figure 1a shows the 5V output 11.4W standard

application circuit and Figure 1b shows the 3.3V output

7.5W standard application circuit. Most applications

will be served by these circuits. To learn more about

component selection for particular applications, refer to

the

Design Procedure section. To learn more about the

operation of the MAX747, refer to the

Detailed

Description.

_______________Detailed Description

The MAX747 monolithic, CMOS, step-down switch-

mode power-supply controller drives external

P-channel FETs. It uses a unique current-mode pulse-

width-modulating (PWM) control scheme that results in

high efficiency over a wide range of load currents, tight

output voltage regulation, excellent load- and line-

transient response, and low noise. Efficiency at light

loads is further enhanced by a proprietary Idle-Mode

switching control scheme that skips oscillator cycles in

order to reduce switching losses.