Rainbow Electronics MAX773 - Manual (Page 9)

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
MAX770–MAX773
5V/12V/15V or Adjustable, High-Efficiency,
Low I Q , Step-Up DC-DC Controllers
_______________________________________________________________________________________ 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