ST777/778/779
LOW VOLTAGE INPUT, 3-3.3V/5V/ADJUSTABLE OUTPUT
DC-DC CONVERTER WITH SYNCHRONOUS RECTIFIER
s
s
s
s
s
s
s
s
s
1V TO 6V INPUT GUARANTEES START-UP
UNDER LOAD
MAXIMUM OUTPUT CURRENT OF 300mA
(778 OR 779 ADJUSTED TO 3V)
LOAD FULLY DISCONNECTED IN
SHUTDOWN
TYPICAL EFFICIENCY OF 82%
INTERNAL 1A POWER SWITCH AND
SYNCHRONOUS RECTIFIER
ADJUSTABLE CURRENT LIMIT ALLOWS
LOW-COST INDUCTORS
SUPPLY CURRENT OF 270µA (NO LOAD)
SHUTDOWN SUPPLY CURRENT 20µA
PACKAGE AVAILABLE: DIP-8 AND SO-8
DIP-8
SO-8
DESCRIPTION
The ST777/778/779 are dc-dc converters that
step-up from low voltage inputs requiring only
three external components, an inductor (typically
22µH) and two capacitors. The device include a
Sinchronous Rectifier that eliminates the need for
an external catch diode, and allows regulation
even when the input is greater than the output.
SCHEMATIC DIAGRAM
V
IN
100µF
Unlike others step-up DC-DC converters the
ST777/778/779’s Sinchronous Rectifier turns off in
the shutdown mode, fully disconnecting the output
from the source. This eliminates the current drain
associated with conventional step-up converters
when off or in shutdown. Supply current is 270µA
under no load and only 20µA in stand by mode.
22µH
R
LIM
2
IN
DELAY
TIMER
1
5
I
LIM
L
X
ACTIVE RECTIFIER
t
OFF
SWITCH
DRIVER
DELAY
TIMER
OUT 6
V
O
100µF
t
ON
1 : N
RECTIFIER
CONTROL
PGND 4
SEL 8
VREF
1.25V
7 SHDN
SHUTDOWN
CONTROL
AGND
3
October 2002
1/11
STMICROELECTRONICS [ STMICROELECTRONICS ]
