5
LTC1645
1645fa
V
CC2
(Pin 1/Pin 1): Positive Supply Input. V
CC2
can range
from 1.2V to 12V for normal operation. I
CC2
is typically
0.2mA. An undervoltage lockout circuit disables the
LTC1645 whenever the voltage at V
CC2
is less than 1.12V.
SENSE2 (Pin 2/Pin 2): V
CC2
Circuit Breaker Set Pin. With
a sense resistor placed in the supply path between V
CC2
and SENSE2, the circuit breaker trips when the voltage
across the resistor exceeds 50mV for more than 1.5µs. If
the circuit breaker trip current is set to twice the normal
operating current, only 25mV is dropped across the sense
resistor during normal operation. To disable the circuit
breaker, short V
CC2
and SENSE2 together.
GATE2 (Pin 3/Pin 3): Channel 2 High Side Gate Drive.
Connect to the gate of an external N-channel MOSFET. An
internal charge pump guarantees at least 4.5V of gate
drive. The charge pump is powered by the higher of V
CC1
and V
CC2
. When the ON pin exceeds 2V, GATE2 is turned
on by connecting a 10µA current source from the charge
pump output to the GATE2 pin and the voltage starts to
ramp up with a slope dv/dt = 10µA/C
GATE2
. While the ON
pin is below 2V but above 0.4V, a 40µA current source
pulls GATE2 toward ground. If the ON pin is below 0.4V,
the circuit breaker trips or the undervoltage lockout circuit
trips, the GATE2 pin is immediately pulled to ground with
a 12mA (typ) current source.
FAULT (Pin 4/NA): Circuit Breaker Fault. FAULT is an
open-drain output that pulls low when the circuit breaker
function trips. The circuit breaker is reset by pulling the ON
pin below 0.4V. An external pull-up is required to generate
a logic high at the FAULT pin. When the ON pin is low,
FAULT will release.
The circuit breaker can be programmed to automatically
reset by connecting the FAULT pin to the ON pin. In this
circuit configuration, if a logic device is driving the ON pin,
use a series resistor between the logic output and the ON
pin to prevent large currents from flowing.
RESET (Pin 5/NA): Open-Drain RESET Output. The RESET
pin is pulled low when the voltage at the FB pin goes below
1.238V or V
CC1
is below the undervoltage lockout thresh-
old. The RESET pin goes high one timing cycle after the
voltage at the FB pin goes above the FB pin threshold. The
ON pin must remain above 0.8V during this timing cycle.
An external pull-up is required to generate a logic high at
the RESET pin.
FB (Pin 6/NA): RESET Comparator Input. The FB pin is
used to monitor the output supply voltage with an external
resistive divider. When the voltage on the FB pin is lower
than 1.238V, the RESET pin is pulled low. A glitch filter on
the FB pin prevents fast transients from forcing RESET
low. When the voltage on the FB pin rises above the trip
point, the RESET pin goes high after one timing cycle.
GND (Pin 7/Pin 4): Ground. Connect to a ground plane for
optimum performance.
COMP
+
(Pin 8/NA): Spare Comparator Noninverting In-
put. When the voltage on COMP
+
is lower than 1.238V,
COMPOUT pulls low.
COMPOUT (Pin 9/NA): Open-Drain Spare Comparator
Output. COMPOUT pulls low when the voltage on COMP
+
is below 1.238V or V
CC1
is below the undervoltage lockout
threshold. An external pull-up is required to generate a
logic high at the COMPOUT pin.
ON (Pin 10/Pin 5): Analog Control Input. If the ON pin
voltage is below 0.4V, both GATE1 and GATE2 are imme-
diately pulled to ground. While the voltage is between 0.4V
and 0.8V, both GATE1 and GATE2 are each pulled to
ground with a 40µA current source. While the voltage is
between 0.8V and 2V, the GATE1 pull-up is turned on after
one timing cycle, but GATE2 continues to be pulled to
ground with a 40µA current source. When the voltage
exceeds 2V, both the GATE1 and GATE2 pull-ups are
turned on one timing cycle after the voltage exceeds 0.8V.
The ON pin is also used to reset the electronic circuit
breaker. If the ON pin is brought below and then above
0.4V following the trip of the circuit breaker, the circuit
breaker resets, and a normal power-up sequence occurs.
TIMER: (Pin 11/NA): System Timing Pin. The TIMER pin
requires an external capacitor to ground to generate a
timing delay. The pin is used to set the delay before the
RESET pin goes high after the output supply voltage is
good as sensed by the FB pin. It is also used to set the delay
between the ON pin exceeding 0.8V and the GATE1 and
GATE2 pins turning on (GATE2 turns on only if the ON pin
exceeds 2V).
(14-Lead Package/8-Lead Package)
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