LT1947
5
1947fa
FB1 (Pin 1): Feedback Pin for First Switcher. Connect
resistor divider tap here. Set AV
DD
according to:
AV
DD
= 1.26V(1 + R1/R2).
FB2 (Pin 2): Feedback Pin for Second Switcher. Connect
resistor divider 2 here and set V
ON
using:
V
ON
= 1.26V (1 + R3/R4) – 160mV.
C
T
(Pin 3): Timing Capacitor Pin. Connect a 10nF capacitor
from C
T
to ground to program a 2.3ms delay from FB1
reaching 1.26V to V
ON
turning on.
SW1 (Pin 4): AV
DD
Switch Node. Connect L1 and D1 here
(see Figure 1). Minimize trace area at this pin to keep EMI
down.
GND (Pin 5): Ground. Connect directly to local ground
plane.
V
IN
(Pin 6): Input Supply Pin. Must be bypassed with a
ceramic capacitor close to the pin.
SW2 (Pin 7): V
O2
Switch Node. Connect L2 and D2 here.
Minimize trace area at this pin to keep EMI down.
SHDN (Pin 8): Pull this pin low for shutdown mode. For
normal operation, tie to a voltage between 2.4V and 8V.
V
O2
(Pin 9): SW2 Output. This node is also internally
connected to the emitter of Q3 (see Block Diagram), the
high side switch between V
O2
and V
ON
.
V
ON
(Pin 10): This is the delayed output for second
Switcher. V
ON
reaches its programmed voltage after the
internal timer times out.
Exposed Pad (Pin 11): Ground (MSE package only). The
exposed pad must be soldered to the PCB and electrically
connected to ground.
UU
U
PI FU CTIO S
