Datasheet

LT4256-3
7
42563fa
PI FU CTIO S
UUU
GATE (Pin 13): High Side Gate Drive for the External
N-Channel MOSFET. An internal charge pump guarantees
at least 10V of gate drive for V
CC
supply voltages above
20V and 4.5V of gate drive for V
CC
supply voltages
between 10.8V and 20V. The rising slope of the voltage on
GATE is set by an external capacitor connected from GATE
to GND and an internal 32µA pull-up current source from
the charge pump output.
If the current limit is reached, the GATE voltage is adjusted
to maintain a constant voltage across the sense resistor
while the timing capacitor starts to charge. If the TIMER
voltage ever exceeds 4.65V, GATE is pulled low.
GATE is also pulled to GND whenever UV is pulled low; the
V
CC
supply voltage drops below the externally programmed
undervoltage threshold, above the overvoltage threshold
or below the internal UVLO threshold (9.8V).
GATE is clamped internally to a maximum voltage of
11.6V (typ) above V
OUT
under normal operating condi-
tions. Driving this pin beyond the clamp voltage may
damage the part. GATE is also clamped to 2V (typ) below
V
OUT
. When the gate is commanded off due to a fault
condition, it is discharged quickly by a 62mA (typ)
capable switch until GATE is 2V (typ) below V
OUT
. When
GATE is below V
OUT
by 2V, the 62mA is reduced to 130µA
to protect the LT4256-3 against damage if V
OUT
has large
capacitance. A Zener diode is needed between the gate
and source of the external MOSFET to protect its gate
oxide under instantaneous short-circuit conditions. See
Applications Information.
SENSE (Pin 15): Current Limit Sense Input. A sense
resistor is placed in the supply path between V
CC
and
SENSE. The current limit circuit regulates the voltage
across the sense resistor (V
CC
– SENSE) to 55mV while in
current limit when FB is 2V or higher. If FB drops below
2V, the regulated voltage across the sense resistor de-
creases linearly and stops at 14mV when FB is 0V. The
OPEN output also uses SENSE to detect when the output
current is less than (3mV)/R5.
To defeat current limit, connect SENSE to V
CC
.
V
CC
(Pin 16): Input Supply Voltage. The positive supply
input ranges from 10.8V to 80V for normal operation. I
CC
is typically 1.8mA. An internal circuit disables the LT4256-3
for inputs less than 9.8V (typ).