_______________Detailed Description

The MAX749 is a negative-output inverting power con-

troller that can drive an external PNP transistor or P-

channel MOSFET. An external resistor and an internal

DAC control the output voltage (Figure 1).

The MAX749 is designed to operate from 2V to 6V inputs,

ideal for operation from low-voltage batteries. In systems

with higher-voltage batteries, such as notebook comput-

ers, the MAX749 may also be operated from the regulat-

ed +5V supply. A high-efficiency +5V regulator, such as

the MAX782, is an ideal source for the MAX749. In this

example, the MAX749 efficiency (80%) is compounded

with the MAX782 efficiency (95%): 80% x 95% = 76%,

which is still high.

Operating Principle

The MAX749 and the external components shown in the

Typical Operating Circuit form a flyback converter.

When the external transistor is on, current flows through

the current-sense resistor, the transistor, and the coil.

Energy is stored in the core of the coil during this phase,

and the diode does not conduct. When the transistor

turns off, current flows from the output through the diode

and the coil, driving the output negative. Feedback con-

trol adjusts the external transistor’s timing to provide a

regulated negative output voltage.

The MAX749’s unique control scheme combines the

ultra-low supply current of pulse-skipping, pulse-fre-

quency modulation (PFM) converters with the high full-

load efficiency characteristic of pulse-width modulation

(PWM) converters. This control scheme allows the

device to achieve high efficiency over a wide range of

loads. The current-sense function and high operating

frequency allow the use of tiny external components.

Switching control is accomplished through the combi-

nation of a current limit in the switch plus on- and off-

time limits (Figure 2).

Once turned on, the transistor stays on until either:

- the maximum on-time one-shot turns it off

(8µs later), or

- the switch current reaches its limit (as determined

by the current-sense resistor and the current

comparator).

Digitally Adjustable LCD Bias Supply

_______________________________________________________________________________________

5

L1

47

µH

DLOW

SWITCH-

MODE

POWER

SUPPLY

6-BIT

CURRENT-OUTPUT

DAC

REF

LOGIC

BIAS

POWER-ON

RESET

6-BIT

COUNTER

MAX749

(NEGATIVE)

CTRL

ADJ

RESET

INCREMENT

ON/OFF

+2V TO +6V

INPUT

22

µF

6.2V

6.66

µA TO 20µA

CS

DHI

FB

GND

Q1

ZTX750

D1

1N5819

470

Ω

22

µF

30V

V+

0.1

µF

Figure 1. Block Diagram, Showing External Circuitry Using a PNP Transistor