Floating the shunt-regulator ground (SGND) disables

the shunt regulator. To enable it, connect SGND to

GND. The shunt regulator requires 1mA minimum cur-

rent for proper operation; the maximum current must

not exceed 20mA. The MAX773 operates in non-boot-

strapped mode when the shunt regulator is used, and

EXT swings between the 6V shunt-regulator voltage

and GND.

When using the shunt regulator, use an N-channel pow-

er FET instead of an NPN power transistor as the power

switch. Otherwise, excessive base drive will collapse

the shunt regulator.

External Power-Transistor

Control Circuitry

PFM Control Scheme

The MAX770–MAX773 use a proprietary current-limited

PFM control scheme to provide high efficiency over a

wide range of load currents. This control scheme com-

bines the ultra-low supply current of PFM converters (or

pulse skippers) with the high full-load efficiency of

PWM converters.

Unlike traditional PFM converters, the MAX770–

MAX773 use a sense resistor to control the peak induc-

tor current. They also operate with high switching

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

_______________________________________________________________________________________

9

V+

EXT

CONTROL

1.5V

REFERENCE

Q

TRIG

Q

S

F/F

R

Q

TRIG

LOW-VOLTAGE

OSCILLATOR

2.5V

0.1V

0.2V

ONE-SHOT

ONE-SHOT

CURRENT-SENSE

AMPLIFIER

DUAL-MODE

COMPARATOR

LBO V15

V12

V5

FB

LBI

REF

200mV

ERROR

COMPARATOR

SHDN

V+

SGND

6V

EXTH

EXTL

EXT

CS

MAX773 ONLY

MAX770

MAX771

MAX772

MAX770

MAX771

MAX772

BIAS

CIRCUITRY

N

N

N

MAX770–MAX773

MAX773

ONLY

Figure 1. Block Diagram