Datasheet

LT3837
7
3837fd
PIN FUNCTIONS
V
C
(Pin 9): Pin used for frequency compensation for the
switcher control loop. It is the output of the feedback
amplifier and the input to the current comparator. Switcher
frequency compensation components are normally placed
on this pin to ground. The voltage on this pin is propor-
tional to the peak primary switch current. The feedback
amplifier output is enabled during the synchronous switch
on time.
UVLO (Pin 10): A resistive divider from V
IN
to this pin sets
an undervoltage lockout based upon V
IN
level (not V
CC
).
When the UVLO pin is below its threshold, the gate drives
are disabled, but the part draws its normal quiescent current
from V
CC
. The V
CC
undervoltage lockout supersedes this
function so V
CC
must be great enough to start the part.
The bias current on this pin has hysteresis such that the
bias current is sourced when the UVLO threshold is ex-
ceeded. This introduces a hysteresis at the pin equivalent
to the bias current change times the impedance of the
upper divider resistor. The user can control the amount
of hysteresis by adjusting the impedance of the divider.
See the Applications Information section for details. Tie
the UVLO pin to V
CC
if you are not using this function.
SENSE
–
(Pin 11), SENSE
+
(Pin 12): These pins are used to
measure primary side switch current through an external
sense resistor. Peak primary side current is used in the
converter control loop. Make Kelvin connections to the
sense resistor to reduce noise problems. SENSE
–
con-
nects to the ground side. At maximum current (V
C
at its
maximum voltage) it has a 98mV threshold. The signal is
blanked (ignored) during the minimum turn-on time.
C
CMP
(Pin 13): Pin for external filter capacitor for the
optional load compensation function. Load compensation
reduces the effects of parasitic resistances in the feedback
sensing path. A 0.1µF ceramic capacitor suffices for most
applications. Short this pin to GND in less demanding ap-
plications that don’t require load compensation.
R
CMP
(Pin 14): Pin for optional external load compensation
resistor. Use of this pin allows for nominal compensation
of parasitic resistances in the feedback sensing path. In
less demanding applications, this resistor is not needed
and this pin can be left open. See the Applications Infor-
mation section for details.
PGDLY (Pin 15): Pin for external programming resistor
to set delay from synchronous gate turn-off to primary
gate turn-on. See the Applications Information section
for details.
PG (Pin 16): Gate drive pin for the primary side MOSFET
Switch. Large dynamic currents flow during voltage
transitions. See the Applications Information section for
details.
GND (Exposed Pad Pin 17): This is the ground connec-
tion for both signal ground and gate driver grounds. This
GND should be connected to the PCB ground plane for
electrical contact and rated thermal performance. Careful
attention must be paid to ground layout. See the Applica-
tions Information section for details.