LT1640ALCS8 datasheet(8/16 Pages) LINER

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LT1640AL/LT1640AH

Capacitor C1 and resistor R3 prevent Q1 from momen-

tarily turning on when the power pins first make contact.

Without C1 and R3, capacitor C2 would pull the gate of Q1

up to a voltage roughly equal to VEE • C2/CGS(Q1) before

the LT1640A could power up and actively pull the gate

low. By placing capacitor C1 in parallel with the gate

capacitance of Q1 and isolating them from C2 using

resistor R3, the problem is solved. The value of C1 should

be:

INMAX

−









•+

()

where VTH is the MOSFET’s minimum gate threshold and

VINMAX is the maximum operating input voltage.

R3’s value is not critical and is given by (VINMAX + ∆VGATE)/

5mA.

The waveforms are shown in Figure 6b. When the power

pins make contact, they bounce several times. While the

contacts are bouncing, the LT1640A senses an

undervoltage condition and the GATE is immediately

pulled low when the power pins are disconnected.

Once the power pins stop bouncing, the GATE pin starts to

ramp up. When Q1 turns on, the GATE voltage is held

constant by the feedback network of R3 and C2. When the

DRAIN voltage has finished ramping, the GATE pin then

ramps to its final value.

Figure 6b. Inrush Control Waveforms

1640A F07b

CONTACT

BOUNCE

APPLICATIO S I FOR ATIO

LT1640ALCS8 Datasheet(PDF) 8 Page - Linear Technology