LTC1646IGN datasheet(7/20 Pages) LINER

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LTC1646

1646fa

PI FU CTIO S

3VSENSE (Pin 9): 3.3V Current Limit Set. With a sense

resistor placed in the supply path between 3VIN and

3VSENSE, the GATE pin voltage will be adjusted to maintain

a constant voltage across the sense resistor and a con-

stant current through the switch while the TIMER pin

voltage is less than 1.25V. A foldback feature makes the

current limit decrease as the voltage at the 3VOUT pin

approaches GND.

When the TIMER pin voltage exceeds 1.25V, the circuit

breaker function is enabled. If the voltage across the sense

resistor exceeds 56mV, the circuit breaker is tripped after

a 21

µs time delay. In the event the sense resistor voltage

exceeds 150mV, the circuit breaker trips immediately and

the chip latches off. To disable the 3.3V current limit,

3VSENSE and 3VIN can be shorted together.

GATE (Pin 10): High Side Gate Drive for the External 3.3V

and 5V 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 13

µA

current source connected to the internal charge pump and

the external capacitor connected to GND or by the 3.3V or

5V current limit and the bulk capacitance on the 3VOUT or

5VOUT supply lines. During power-down, the slope of the

ramp down voltage is set by the 200

µA 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 while the TIMER pin voltage is less than

1.25V. If a current fault occurs after the TIMER pin voltage

exceeds 1.25V, the GATE pin is immediately pulled to

GND.

5VSENSE (Pin 11): 5V Current Limit Set. With a sense

resistor placed in the supply path between 5VIN and

5VSENSE, the GATE pin voltage will be adjusted to maintain

a constant voltage across the sense resistor and a con-

stant current through the switch while the TIMER pin

voltage is less than 1.25V. A foldback feature makes the

current limit decrease as the voltage at the 5VOUT pin

approaches GND.

When the TIMER pin voltage is greater than 1.25V, the

circuit breaker function is enabled. If the voltage across

the sense resistor exceeds 56mV but is less than 150mV,

the circuit breaker is tripped after a 21

µs time delay. In the

event the sense resistor voltage exceeds 150mV, the

circuit breaker trips immediately and the chip latches off.

To disable the 5V current limit, short 5VSENSE and 5VIN

together.

5VIN (Pin 12): 5V Supply Sense Input. An undervoltage

lockout circuit prevents the GATE pin voltage from

ramping up when the voltage at the 5VIN pin is less than

2.5V. If no 5V input supply is available, tie the 5VIN pin to

the 3VIN pin.

PRECHARGE (Pin 13): Precharge Monitor Input. An on-

chip error amplifier with a 1V reference servos the DRIVE

pin voltage to keep the precharge node at 1V. If the

precharge function is not being used, tie the PRECHARGE

pin to GND.

DRIVE (Pin 14): Precharge Base Drive Output. Provides

base drive for an external NPN emitter-follower which in

turn biases the PRECHARGE node. If the precharge func-

tion is not being used, allow the DRIVE pin to float.

OFF/ON (Pin 15): Digital Input. Connect the CPCI BD_SEL#

signal to the OFF/ON pin. When the OFF/ON pin is pulled

low, the GATE pin is pulled high by a 13

µA current source.

When the OFF/ON pin is pulled high the GATE pin will be

pulled to ground by a 200

µA current source.

The OFF/ON pin is also used to reset the electronic circuit

breaker. If the OFF/ON pin is cycled high and low following

the trip of the circuit breaker, the circuit breaker is reset,

and a normal power-up sequence will occur.

RESETIN (Pin 16): Digital Input. Connect the CPCI

PCI_RST# signal to the RESETIN pin. Pulling RESETIN low

will cause the RESETOUT pin to pull low.

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