LTC4269-1
10
42691fc
the V
CMP
voltage and thus limits peak current until soft-
start is complete. The ramp time is approximately 70ms
per µF of capacitance. Leave SFST open if not using the
soft-start function.
OSC (Pin 15): Oscillator. This pin, in conjunction with an
external capacitor (C
OSC
) to GND, defi nes the controller
oscillator frequency. The frequency is approximately
100kHz • 100/C
OSC
(pF).
FB (Pin 16): Feedback Amplifi er Input. Feedback is usually
sensed via a third winding and enabled during the fl yback
period. This pin also sinks additional current to compensate
for load current variation as set by the R
CMP
pin. Keep the
Thevenin equivalent resistance of the feedback divider at
roughly 3k.
V
CMP
(Pin 17): Frequency Compensation Control. V
CMP
is used for frequency compensation of the switcher con-
trol loop. It is the output of the feedback amplifi er and
the input to the current comparator. Switcher frequency
compensation components are placed on this pin to GND.
The voltage on this pin is proportional to the peak primary
switch current. The feedback amplifi er output is enabled
during the synchronous switch on time.
UVLO (Pin 18): Undervoltage Lockout. A resistive divider
from V
PORTP
to this pin sets an undervoltage lockout based
upon V
PORTP
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 UVLO threshold is exceeded.
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. Tie the UVLO
pin to V
CC
if not using this function. See the Applications
Information section for details. This pin is used for the
UVLO function of the switching regulator. The PD interface
section has an internal UVLO.
SENSE
–
, SENSE
+
(Pins 19, 20): Current Sense Inputs.
These pins are used to measure primary-side switch cur-
rent through an external sense resistor. Peak primary-side
PIN FUNCTIONS
current is used in the converter control loop. Make Kelvin
connections to the sense resistor R
SENSE
to reduce noise
problems. SENSE
–
connects to the GND side. At maximum
current (V
CMP
at its maximum voltage) SENSE pins have
100mV threshold. The signal is blanked (ignored) during
the minimum turn-on time.
C
CMP
(Pin 21): Load Compensation Capacitive Control.
Connect a capacitor from C
CMP
to GND in order to reduce
the effects of parasitic resistances in the feedback sensing
path. A 0.1µF ceramic capacitor suffi ces for most applica-
tions. Short this pin to GND when load compensation is
not needed.
R
CMP
(Pin 22): Load Compensation Resistive Control.
Connect a resistor from R
CMP
to GND in order to com-
pensate for 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
Information section for details.
PGDLY (Pin 23): Primary Gate Delay Control. Connect an
external programming resistor (R
PGDLY
) to set delay from
synchronous gate turn-off to primary gate turn-on. See
the Applications Information section for details.
PG (Pin 24): Primary Gate Drive. PG is the gate drive pin
for the primary-side MOSFET switch. Large dynamic cur-
rents fl ow during voltage transitions. See the Applications
Information section for details.
V
NEG
(Pins 26, 27): System Negative Rail. Connects V
NEG
to V
PORTN
through an internal power MOSFET. Pin 26 and
Pin 27 must be electrically tied together at the package.
PWRGD (Pin 29): Power Good Output, Open-Collector.
High impedence signals power-up completion. PWRGD
is referenced to V
NEG
and features a 14V clamp.
PWRGD (Pin 30): Complementary Power Good Output,
Open-Drain. Low impedance signals power-up completion.
PWRGD is referenced to V
PORTN
.
V
PORTP
(Pin 32): Positive Power Input. Tie to the input
port power through the input diode bridge.
Exposed Pad (Pin 33): Ground. This is the negative rail
connection for both signal ground and gate driver grounds
of the fl yback controller. This pin should be connected to
V
NEG
.
