LTC3725
6
3725fa
UU
U
PI FU CTIO S
UVLO (Pin 1): Undervoltage Lockout. Connect to a resis-
tive voltage divider to monitor input voltage V
IN
. Enables
converter operation for V
UVLO
> 1.242V. Hysteresis is a
fixed 16mV hysteresis voltage with a 4.9µA hysteresis
current that combines with the Thevenin resistance of the
divider to set the total UVLO hysteresis voltage. This input
also senses V
IN
for voltage feedforward. Finally, this pin
can be used for external run/stop control.
SSFLT (Pin 2): Combination Soft-Start and Fault Indica-
tor. A capacitor to GND sets the duty cycle ramp-up rate
during start-up. To indicate a fault, the SSFLT pin is
momentarily pulled up to within 1.3V of V
CC
.
NDRV (Pin 3): Drive for the External NMOS Linear Regu-
lator. Connect to the gate of the NMOS and connect a pull
up resistor to the input voltage (V
IN
). Optionally, to create
a trickle charger omit the NMOS device and connect NDRV
to V
CC
.
FB/IN
+
(Pin 4): This pin has several functions. One wind-
ing of a pulse transformer is connected to the FB/IN
+
and
FS/IN
–
pins. The other pulse transformer winding is con-
nected to the LTC3706. The LTC3725 automatically de-
tects when the LTC3706 applies a pulse-encoded signal to
the FB/IN
+
and FS/IN
–
pins and decodes duty cycle infor-
mation for control of the primary-side gate drive (see
Operation below). In secondary-side control, primary-
side gate drive bias power is also extracted from the FB/IN
+
and FS/IN
–
pins using an on-chip full-wave rectifier.
For primary-side control connect this pin to an optoisolator
for feedback control of converter output voltage using an
internal optoisolator biasing network.
FS/IN
–
(Pin 5): This pin has several functions. Place a
resistor from this pin to GND to set the oscillator fre-
quency. For secondary-side control with the LTC3706,
connect one winding of the pulse transformer for opera-
tion as described for the FB/IN
+
pin above.
PGND (Pin 6): Supply Return for the Bottom Gate Driver
and the On-Chip Bridge Rectifier.
GATE (Pin 7): Gate Drive. Connect to the gate of the
external MOSFET.
V
CC
(Pin 8): Main V
CC
Power for All Driver and Control
Circuitry.
V
SLMT
(Pin 9): Volt-Second Limit. Form an R-C integrator
by connecting a resistor from V
IN
to V
SLMT
and a capacitor
from V
SLMT
to ground. The gate drive is turned off when
the voltage on the V
SLMT
pin exceeds 1.26V.
I
S
(Pin 10): Input to the Overcurrent Comparator. Connect
to the positive terminal of a current-sense resistor in
series with the source of the ground-referenced bottom
MOSFET.
GND (Pin 11): Signal Ground.
