Datasheet
LTC2923
10
2923fa
APPLICATIO S I FOR ATIO
WUUU
Shutdown Output
In some applications it might be necessary to control the
shutdown or RUN/SS pins of the slave supplies using the
12-lead LTC2923CDE or LTC2923IDE. The LTC2923 may
not be able to supply the rated 1mA of current from the FB1
and FB2 pins when V
CC
is below 2.9V. If the slave power
supplies are capable of operating at low input voltages,
use the open-drain SDO output to drive the SHDN or
RUN/SS pins of the slave supplies (see Figure 7). This will
hold the slave supplies’ outputs low until the ON pin is
above 1.23V, V
CC
is above the 2.6V undervoltage lockout
condition and there are no short-circuit faults latched. It
pulls low again when the ON pin is pulled below 1.23V and
the RAMP pin is below about 100mV. When two supplies
must have their RUN/SS or SHDN pins controlled indepen-
dently, tie a Schottky diode between each pin and the SDO
output (see Figure 8).
Status Output
The STATUS pin provides an indication that the supplies
are finished ramping up. This pin is an open-drain output
that pulls low until the GATE has been fully charged. Since
the GATE pin drives the gate of the external FET, or the
RAMP pin directly when no FET is used, the supplies are
completely ramped up when the GATE pin is fully charged.
It will go low again when the GATE pin is pulled low, either
because of a short-circuit fault or because the ON pin has
been pulled low.
Q1
C
GATE
10nF
0.1µF
10Ω
R
STATUS
10k
V
CC
V
IN
V
IN
V
IN
R
ONB
138k
V
IN
3.3V
R
TB1
16.5k
R
TB2
887k
R
FA1
35.7k
1.8V
3.3V
R
FB1
16.5k
R
TA2
412k
R
TA1
13k
R
ONA
100k
RAMPBUF
TRACK1
TRACK2
FB1
GATE
LTC2923
GND
2923 F07
RAMP
R
FA2
412k
2.5V
R
FB2
887k
DC/DC
IN
FB = 1.235V OUT
DC/DC
IN
FB = 0.8V
RUN/SS
RUN/SS
OUT
FB2
SDO
STATUS
ON
Q1
C
GATE
10nF
R
STATUS
10k
V
CC
V
IN
V
IN
V
IN
R
ONB
138k
V
IN
3.3V
R
TB1
16.5k
R
TB2
887k
R
FA1
35.7k
1.8V
3.3V
R
FB1
16.5k
R
TA2
412k
R
TA1
13k
R
ONA
100k
RAMPBUF
TRACK1
TRACK2
FB1
GATE
LTC2923
GND
2923 F08
RAMP
R
FA2
412k
2.5V
R
FB2
887k
DC/DC
IN
FB = 1.235V OUT
DC/DC
IN
FB = 0.8V
RUN/SS
RUN/SS
OUT
FB2
SDO
STATUS
ON
0.1µF
10Ω
Figure 7 Figure 8