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UC1843 failing to startup

Doug Wendt
Prodigy 70 points

No gate driver operation. Anyone have any ideas?

Controller was operating with fairly large  capacitor on VCC, 22uF. Attempted reducing size to reduce delay time to start. Now, after VCC rises to UVLO turn ON level of 8.4V, oscillator starts but no gate driver output. Subsequently oscillator and internal loading drains charge on VCC capacitor till turn OFF level of 7.6V. Cycle repeats.

Changed capacitor back to 22uF, then larger, with no change. Nothing at Gate driver output.

Replaced IC, and same issue with 22uF on VCC.

Anyone experience this?

  • 0
    TI__Mastermind 34465 points
    The PWM should start switching if the FB pin is below 2.5V and ISENSE is below 1V.
    Im not clear about your last sentence.
    When you replaced the IC and reverted to 22uF does the controller operate?
    or did just the oscillator operate?
    Is the MOSFET damaged?
  • 0 in reply to John Griffin1
    Prodigy 70 points
    John,

    Got it working.
    Agreed, it should start. Regarding last sentence, with new controller same thing, oscillator working but no gate drive output. Mosfet was fine.
    Eventually started looking into the shutdown mechanisms. (i.e. Isense too high or Comp pulled to GND or impedance too low)
    Started disconnecting things, when I disconnected the feedback network I started getting gate driver pulses.
    Resolution: Increased impedance of feedback and compensation network. Original network had 10K from Vout (bootstrap) to FB, and 2K from FB to GND. From example circuit in datasheet the resistor from FB to GND was 4.7K, Based on this I increased the two resistors to 25.5K and 5.1K respectively and all is good.