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LM3671 Startup Issue, 8V spike when enabled

Anthony Woodward
Prodigy 210 points
Other Parts Discussed in Thread: LM3671

I have 5V coming in, then through a Schottky, so the regulator sees about 4.7V and is very steady. Then as an RC filter at EN gets to about 1V, VIN starts to oscillate between 4.7V and 2V, and VOUT jumps to 2V. This goes on for 25ms to 100ms, at which point VIN spikes to 8V for about 500us as VOUT goes to 3.3V, then everything settles out, with VIN remaining at 4.7V.

Why the spike, and what to do about it? It's not coming from the source, and there's nothing else in the circuit. I have many boards that work, but 2 boards where the regulator appears to have died (input DC resistance is now about 150 ohms when off, and 0V on VOUT when powered). The absolute maximum rating of LM3671 VIN is 6V.

Our board only consumes about 300mA, and the design is good for at least 600mA.

Thanks,

- Tones

  • 0
    Prodigy 210 points
    Correction... Schottky, not a Zener.
  • 0 in reply to Anthony Woodward
    TI__Expert 3340 points
    Hi, can you share a schematic of your design please?
  • 0
    Intellectual 510 points

    It would be good to see your schematic and to know the part number of the schottky diode you have used. Also what input capacitance you have.

    The LM3761 starts up with a staged current limit during which the input current will be rising in steps as the circuit output starts up. The startup current limit may hit 0.6A or so even though your run-time load current is less.

    Your schottky may have a wide vf vs current spec so if this startup current is being taken via this diode it could be causing the drop. The problem may be that your input enable is derived from the supply line so as it turns on the supply voltage drops due to the startup current, which then dips below the enable threshold turning it off, allowing it to rise and enable again potentially causing an oscillation.

    A voltage detector with suitable hysteresis may prevent this rather than an RC circuit. I used a voltage monitor IC to turn the enable on when VCC reaches 3.0V. These devices are typically £0.15 so not too expensive.

    Mark

  • 0 in reply to Mark Williams
    Intellectual 510 points

    The screenshot below was my startup (before adding the voltage monitor). As the 5V rail rises the output is enabled but you can clearly see the staged current limit controlling the output rise.

    This was when I had Vin tied to enable but I have since changed this to use the voltage monitor.