7
LTC1643AL
LTC1643AL-1/LTC1643AH
1643afb
3V
SENSE
(Pin 10): The 3.3V Current Limit Set Pin. With a
sense resistor placed in the supply path between 3V
IN
and
3V
SENSE
, the GATE pin voltage will be adjusted to maintain
a constant voltage across the sense resistor and a con-
stant current through the switch. A foldback feature makes
the current limit decrease as the voltage at the 3V
OUT
pin
approaches GND. To disable the current limit, 3V
SENSE
and 3V
IN
can be shorted together.
GATE (Pin 11): High Side Gate Drive for the External
N-Channel Pass Transistors. Requires an external series
RC network for the current limit loop compensation and
setting the minimum ramp-up rate. During power-up, the
slope of the voltage rise at the GATE is set by the 62µA
current source connected to 12V
IN
and the external ca-
pacitor connected to GND or by the 3.3V or 5V current limit
and the bulk capacitance on the 3V
OUT
or 5V
OUT
suppy
lines. During power-down, the slope of the falling voltage
is set by the 200µA current source connected to GND and
the external GATE capacitor.
The voltage at the GATE pin will be modulated to maintain
a constant current when either the 3V or 5V supplies go
into current limit. When a current limit fault occurs after
the inhibit period set by the TIMER pin capacitance, the
undervoltage lockout circuit on 3.3V, 5V or 12V trips or
the FAULT pin is pulled low, the GATE pin is immediately
pulled to GND.
5V
SENSE
(Pin 12): 5V Current Limit Set Pin. With a sense
resistor placed in the supply path between 5V
IN
and
5V
SENSE
, the GATE pin voltage will be adjusted to maintain
a constant voltage across the sense resistor and a con-
stant current through the switch. A foldback feature makes
the current limit decrease as the voltage at the 5V
OUT
pin
approaches GND. To disable the current limit, 5V
SENSE
and 5V
IN
can be shorted together.
5V
IN
(Pin 13): Analog Input. Used to monitor the 5V input
supply voltage. An undervoltage lockout circuit prevents
the switches from turning on when the voltage at the 5V
IN
pin is less than 2.5V typically.
5V
OUT
(Pin 14): Analog Input. Used to monitor the 5V
output supply voltage. The PWRGD signal cannot go low
until the 5V
OUT
pin exceeds 4.65V typically.
the ON pin pulled low, the GATE pin is pulled high by a
62µA current source and the internal 12V and – 12V
switches are turned on. When the ON pin is pulled low or
the ON pin pulled high, the GATE pin will be pulled to
ground by a 200µA current source and the 12V and – 12V
switches turned off.
The ON/ON pin is also used to reset the electronic circuit
breaker. If the ON/ON pin is cycled following the trip of the
circuit breaker, the circuit breaker is reset and a normal
power-up sequence will occur.
FAULT (Pin 6): Open-Drain Digital I/O. FAULT is pulled low
when a current limit fault is detected. Current limit faults
are ignored while the voltage at the TIMER pin is less than
12V
IN
– 0.9V. Once the TIMER cycle is complete, FAULT
will pull low typically 14.6µs after any of the supplies go
into current limit. At the same time, the GATE and TIMER
pins are pulled to GND and the 12V and –12V switches are
turned off. The chip will remain latched in the off state until
the ON/ON pin is toggled or the power is cycled.
Forcing the FAULT pin low with an external pull-down will
immediately turn off the internal switches and force the
GATE and TIMER pins to GND independent of the state of
the ON/ON pin. However, the chip is not latched into the off
state, so when the FAULT pin is released, the state of the
chip will be determined by the ON pin.
PWRGD (Pin 7): Open-Drain Digital Power-Good Output.
PWRGD remains low while V
12VOUT
≥ 11.4V, V
3VOUT
≥ 3V,
V
5VOUT
≥ 4.75V and V
EEOUT
≤ –10.8V. The LTC1643AL-1
has the power good comparators connected to the 12V
OUT
and V
EEOUT
pins disabled, with only the 3V
OUT
and 5V
OUT
outputs being monitored to generate PWRGD. When one
of the supplies falls below its power-good threshold
voltage, PWRGD will go high after a 15µs deglitching time.
The switches will
not
be turned off when PWRGD goes
high.
GND (Pin 8): Chip Ground.
3V
IN
(Pin 9): 3.3V Supply Sense Input. An undervoltage
lockout circuit prevents the switches from turning on
when the voltage at the 3V
IN
pin is less than 2.5V typically.
If no 3.3V input supply is available, tie 3V
IN
to the 5V
IN
pin.
PIN FUNCTIONS
UUU
