load switch capacitance constraint

Considering using a Load Switch like DigiKey FDC6330LDKR-ND for powering on and off a device at the far end of a cable.

The voltage and current requirements are well within spec however load capacitance is not.

My understanding is that the load capacitance is spec-ed in order to reduce inrush current when the FET is turned on.

Voltages are +/-12V and +/-5V with the currents all being less then an amp.

Has anyone overcome the maximum capacitance restriction in a similar application by adding an inductor? Or is the application all wrong and I should simply use a small relay? (am trying to stick with a solid state solution)


Another solution is to add an RC circuit to the gate of the MOSFET to slow the turn-on, which limits the drain current and reduces the risk of causing a glitch at the supply end. Here from On Semi AND9093/D:

load switch capacitance constraint

The detailed calculations for the value of R1 and C1 are in the application note, but you need to estimate the gfs of the MOSFET.

The part you have chosen also recommends a similar circuit (not quite the same, since it has Q1 and the load switch internally connected)

load switch capacitance constraint

If you go beyond the 1000pF recommendation for C1, the switch will operate more slowly and you'll probably have to significantly derate the current handling capability to account for the transient heating during switching.

Category: power Time: 2014-03-24 Views: 1

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