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I designed a constant current buck DC-DC super-cap charger with the NCP3420 as a dual MOSFET driver. The current is hysteresis controlled and the disable and PWM lines are pulled low on power-up.

The circuit is working, but sometimes the driver fails during power-up with the controls pulled low, shorting out the 12V supply. What would cause this?

Link to NCP3420 Datasheet: https://www.onsemi.com/pub/Collateral/NCP3420-D.PDF

X1-1 and X1-2 are 12V inputs from my lab supply

X2-1 and X2-2 are connected to a super-capacitor

The 100n cap is a ceramic 50V

The input is also decoupled by 4 parallel 100n ceramics + 6800µ electrolytic

The diode is a schottky rated 80V, 500mA

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  • \$\begingroup\$ How is the failure fixed? \$\endgroup\$
    – Andy aka
    Commented Jun 16, 2018 at 10:02
  • \$\begingroup\$ I havent been able to fix it yet. The chip needs to be replaced after the faliure \$\endgroup\$
    – JG97
    Commented Jun 16, 2018 at 10:05
  • \$\begingroup\$ Maybe think about a soft start by adding a capacitor from OD to ground and a 1k resistor feeding that pin from where it is fed from? \$\endgroup\$
    – Andy aka
    Commented Jun 16, 2018 at 10:12
  • \$\begingroup\$ The pin is fed by a pic micro and shold be in high-Z during startup and go to low when the micro starts. Also a valid switch state should not destroy the driver, but the mosfets, since the driver supplies no continuous current \$\endgroup\$
    – JG97
    Commented Jun 16, 2018 at 10:17

1 Answer 1

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Think about the Huge low ESR cap that you are charging .What limits the peak current in Q2 ? Also think about the discharge current peaks in Q1 .Is your hysteric current sensing fast enough on R1 ?

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