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MAX774
(Rev.:2002)

-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-to-DC Controllers

Maxim Integrated 

-5V/-12V/-15V or Adjustable, High-Efficiency,

Low IQ Inverting DC-to-DC Controllers

REF

SHDN

V+

FB

MODE

COMPARATOR

50mV

ERROR

COMPARATOR

MAX774

MAX775

MAX776

OUT

Q TRIG

ONE-SHOT

SQ

N

1.5V

REFERENCE

FROM V+

EXT

TRIG Q

ONE-SHOT

R

CURRENT

COMPARATOR

FROM OUT

CS

Figure 1. Functional Diagram

CURRENT-

CONTROL CIRCUITS

GND

0.2V

(FULL CURRENT)

0.1V

(HALF CURRENT)

FROM

V+

Detailed Description

The MAX774/MAX775/MAX776 are negative-output,

inverting power controllers that can be configured to drive

an external P-channel MOSFET. The output voltages are

preset to -5V (MAX774), -12V (MAX775), or -15V

(MAX776). Additionally, all three parts can be set to any

desired output voltage using an external resistor divider.

The MAX774/MAX775/MAX776 have a unique control

scheme (Figure 1) that combines the advantage of

pulse-skipping, pulse-frequency-modulation (PFM)

converters (ultra-low supply current) with the advan-

tage of pulse-width modulation (PWM) converters (high

efficiency with heavy loads). This control scheme

allows the devices to achieve 85% efficiency with loads

from 5mA to 1A.

As with traditional PFM converters, the external

P-channel MOSFET power transistor is turned on when

the voltage comparator senses that the output is below

the reference voltage. However, unlike traditional PFM

converters, switching is controlled by the combination

of a switch current limit (210mV/RSENSE) and

on-time/off-time limits set by one-shots. Once turned

on, the MOSFET stays on until the 16µs maximum on-

time limit is reached or the switch current reaches its

limit (as set by the current-sense resistor).

Once off, the switch is typically held off for a minimum of

2.3µs. It will stay off until the output drops below the level

determined by VREF and the feedback divider network.

With light loads, the MOSFET switches on for one or

more cycles and then switches off, much like in tradi-

tional PFM converters. To increase light-load efficiency,

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