LT3837
6
3837fd
TEMPERATURE (°C)
–50
t
ED
(ns)
280
300
320
25 75
3837 G20
260
240
–25 0
50 100 125
220
200
R
ENDLY
= 90k
TEMPERATURE (°C)
–50
0
t
PGDLY
(ns)
50
150
200
250
0
50
75
3837 G19
100
–25 25
100 125
300
R
PGDLY
= 16.9k
R
PGDLY
= 27.4k
PIN FUNCTIONS
TYPICAL PERFORMANCE CHARACTERISTICS
SG (Pin 1): Synchronous gate drive output. This pin pro-
vides an output signal for a secondary-side synchronous
switch. Large dynamic currents may flow during voltage
transitions. See the Applications Information section for
details.
V
CC
(Pin 2): Supply voltage pin. Bypass this pin to ground
with a 4.7µF capacitor or more.
t
ON
(Pin 3): Pin for external programming resistor to set
the minimum time that the primary switch is on for each
cycle. Minimum turn-on facilitates the isolated feedback
method. See the Applications Information section for
details.
ENDLY (Pin 4): Pin for external programming resistor to
set enable delay time. The enable delay time disables the
feedback amplifier for a fixed time after the turn-off of the
primary-side MOSFET. This allows the leakage inductance
voltage spike to be ignored for flyback voltage sensing.
See the Applications Information section for details.
SYNC (Pin 5): Pin for synchronizing the internal oscillator
with an external clock. The positive edge on a pulse causes
the oscillator to discharge causing PG to go low (off) and
SG high (on). The sync threshold is typically 1.53V. See
the Applications Information section for details. Tie to
ground if unused.
SFST (Pin 6): This pin, in conjunction with a capacitor to
ground, controls the ramp-up of peak primary current as
sensed through the sense resistor. This is used to control
converter inrush current at start-up. The V
C
pin voltage
cannot exceed the SFST pin voltage, so as SFST increases,
the maximum voltage on V
C
increases commensurately,
allowing higher peak currents. Total V
C
ramp time is ap-
proximately 70ms per µF of capacitance. Leave pin open
if not using the soft-start function.
OSC (Pin 7): This pin in conjunction with an external
capacitor defines the controller oscillator frequency. The
frequency is approximately 100kHz • 100/C
OSC
(pF).
FB (Pin 8): Pin for the feedback node for the power supply
feedback amplifier. Feedback is sensed via a transformer
winding and enabled during the flyback period. This pin
also sinks additional current to compensate for load current
variation as set by the R
CMP
pin. Keep the Thevenin equiva-
lent resistance of the feedback divider at roughly 3k.
PG Delay Time vs Temperature
Enable Delay Time vs Temperature
