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Problem of LM3429

Satoshi
Guru 19645 points
Other Parts Discussed in Thread: LM3429

Please see attached circuit below, and let me know about problem reason.

LM3429_document.pdf


①About resistance of RCT pin

 When customer find RCT resistance value by tester on non-12V input, it was a few MΩ.

 (Datasheet is 36Ω_typ)

 Addition: When 12V input, voltage is the same as Vin(12V).

　⇒Is it normal operation?

②Vcc was be 1V.

 Differ from datasheet spec:6.9V.

 Is there find the reason?

Best regards,
Satoshi

  • 0
    TI__Mastermind 25500 points
    That seems like a damaged IC or a shorted R82 if RCT is at 12V. It should not go above its threshold voltage as listed in the datasheet. IC damage, a solder short, or an excessively high gate charge for the FET are the only reasons VCC would not be in regulation. That FET has a pretty high gate charge, but not too high for the switching frequency used. The schematic looks fine so it's likely a bad IC or some other bad component or manufacturing issue. What is the output voltage/current supposed to be? It looks like the OVP is set very high.
  • 0 in reply to Clinton Jensen
    Guru 19645 points
    Clinton Jensen-san

    Thank you for reply.

    Vout is 55.8V, Iout is 85mA / string.
    (Customer OVP is 76V, is OVP good for a little low?)

    Best regards,
    Satoshi
  • 0 in reply to Satoshi
    TI__Mastermind 25500 points

    The abs max for HSP and HSN is 76V and in the event of an open output there will almost certainly be a significant overshoot before the OVP kicks in. This could damage the IC. I would set it lower, perhaps 10% or 15% above nominal max output voltage rather than right at the IC abs max rating.