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LT4254

4254fb

reaches 4.65V (typ), the GATE pin is pulled low; the TIMER

pull-up current will be turned off and the capacitor is dis-

charged by a 3

µA pull-down current. When the TIMER pin

falls below 0.65V (typ), the GATE pin turns on again if the

RETRY pin is high (if the RETRY pin is low, the UV pin must

be pulsed low to reset the internal fault latch before the

GATE pin will turn on). If the RETRY pin is grounded and

the UV pin is not cycled low, the GATE pin remains latched

off and the TIMER pin will be discharged near ground. The

UV pin must be cycled low after the TIMER pin has dis-

charged below 0.65V (typ) to reset the part.

If the RETRY pin is floating or connected to a voltage above

its 1.2V threshold, the LT4254 automatically restarts after

a current fault. Under an output short-circuit condition, the

LT4254 cycles on and off with a 3% on-time duty cycle.

RETRY: Current Fault Retry. RETRY commands the opera-

tional mode of the current limit. If the RETRY pin is

floating, the LT4254 automatically restarts after a current

fault. If it is connected to a voltage below 0.4V, the part

latches off after a current fault (which requires that the UV

pin be cycled low in order to start normal operation again).

GATE: High Side Gate Drive for the External N-Channel

MOSFET. An internal charge pump guarantees at least 10V

of gate drive for VCC supply voltages above 20V and 4.5V

gate drive for VCC supply voltages between 10.8V and 20V.

The rising slope of the voltage on GATE is set by an external

capacitor connected from the GATE pin to GND and an

internal 35

µA pull-up current source from the charge

pump output.

If the current limit is reached, the GATE pin voltage is

adjusted to maintain a constant voltage across the sense

resistor while the timing capacitor starts to charge. If the

TIMER pin voltage ever exceeds 4.65V, the GATE pin is

pulled low.

The GATE pin is also pulled to GND whenever the UV pin

is pulled low, or the VCC supply voltage drops below the

externally programmed undervoltage threshold or above

the overvoltage threshold.

The GATE pin is clamped internally to a maximum voltage

of 11V (typ) above VCCunder normal operating conditions.

PI FU CTIO S

Driving this pin beyond the clamp voltage may damage the

part. A zener diode is needed between the gate and source

of the external MOSFET to protect its gate oxide under in-

stantaneous short-circuit conditions. See Applications

Information.

SENSE: Current Limit Sense. A sense resistor is placed in

the supply path between VCC and SENSE. The current limit

circuit regulates the voltage across the sense resistor (VCC

– SENSE) to 50mV while in current limit when FB is 2V or

higher. If FB drops below 2V, the regulated voltage across

the sense resistor decreases linearly and stops at 15mV

when FB is 0V. The OPEN output also uses SENSE to detect

when the output current is less than (3.5mV)/R5. To defeat

current limit, connect SENSE to VCC.

PWRGD: Open Collector Output to GND. PWRGD is pulled

low whenever the voltage on FB falls below the high-to-low

threshold voltage. It goes into a high impedance state

when the voltage on FB exceeds the low-to-high threshold

voltage. An external pull-up resistor can pull PWRGD to a

voltage higher or lower than VCC. To disable PWRGD, float

this pin and connect FB to the output voltage.

UV: Undervoltage Sense. UV is an input that enables the

output voltage. When the UV pin is driven above 4V, the

GATE pin starts charging and the output turns on. When

the UV pin goes below 3.6V, the GATE pin discharges and

the output shuts off.

Pulsing the UV pin to ground after a current limit fault cycle

(TIMER pin dischaged to below 0.65V typ) resets the fault

latch (when RETRY pin is low, commanding latch off

operation) and allows the part to turn back on. To disable

UV sensing, connect the pin to VCC through a 10k resistor.

OV: Overvoltage Sense. OV is an input that disables the

output voltage. If OV ever goes above 4V, the GATE pin is

discharged and the output shuts off. When OV goes below

3.6V, the GATE pin starts charging and the output turns

back on. To disable OV sensing, connect pin to ground.

OPEN: Open Circuit Detect Output. This pin is an open

collector output that releases and is pulled high through an

external resistor if the load current is less than (3.5mV/R5).

If not used, leave this pin disconnected.

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