5V/12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controllers
MAX770–MAX773
LBO V15
V12
V5
FB
V+
LBI
N
MAX773 ONLY
N
DUAL-MODE
COMPARATOR
MAX770–MAX773
SHDN
200mV
REF
1.5V
REFERENCE
BIAS
CIRCUITRY
ERROR
COMPARATOR
MAX770
MAX771
MAX772
6V
F/F
S
R
LOW-VOLTAGE
OSCILLATOR
ONE-SHOT
TRIG
Q
CURRENT-SENSE
AMPLIFIER
0.2V
0.1V
EXT
MAX770
MAX771
MAX772
CS
Q
EXT
CONTROL
2.5V
EXTH
MAX773
ONLY
EXTL
SGND
ONE-SHOT
Q
TRIG
N
V+
Figure 1. Block Diagram
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
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9
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
