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TPS65920 is not starting properly

alex alex97
Prodigy 185 points
Other Parts Discussed in Thread: TPS65920, DM3730

Hello. I`m building DM3730 based module, I used TPS65920 as PMIC. Here is a schematics:

I made first PCB (without CPU for now), and TPS65920 did not start. The only voltage I got is 1.5V at VINTDIG.OUT (C86). I noticed, when I touch capacitors near C97 (VREF pin) by finger, TPS starts, all power supplies at nominal voltage, and clocks works too! When TPS is not working, there is no voltage on VREF pin, it looks like high impedance input. Did i miss something? Maybe some pull-ups/pull-downs on control lines?

Vbat voltage is 3.6V (nominal for TPS65920), I tried to adjust up to 3.9V, no effect.

  • 0
    Prodigy 185 points
    Correction: the only working supply 1.5V is VRTC output (C 87), not VINDIG.OUT.
  • 0
    Prodigy 185 points
    I noticed difference between my schematics and others: I have floating JTAG.TCK and JTAG.TDI. On other schematics those pins are grounded. Is it crucial? According to datasheet, those pins have pull-ups.
  • 0 in reply to alex alex97
    TI__Genius 10885 points
    The schematic checklist does recommend grounding those pins, though I'm not sure that this is the source of the issue.
    www.ti.com/.../swca081.pdf

    Does adjusting the capacitance on VREF allow the PMIC to start?
    Can you look at the VREF pin with a scope?
    Is the crystal starting correctly, and is there a possibility that the REF and xtal have a noisy ground?
  • 0 in reply to Richard Solarski
    Prodigy 185 points
    I changed VREF capacitance from 0.1u to 22u with no success.. As for ground: I did not split grounds, my PCB is 8-layer, with 2 solid ground planes without big gaps, I thought this is enough.
    Guess, reason of bad staring - is XTAL. There is no oscillation neither on XTAL pins, nor on 32K_OUT. On 32K_XTAL_IN is about 500 mV. Touching this pin by finger causes dirty oscillation and starting the device. I tried several 32K rezonators with no result. Also I tried to change XTAL caps to 4.7p, 10p, 22p. No result, it`s not starting. But using external generator, applied to 32K_IN, device starts fine. Seems I have to use external oscillator.
  • 0 in reply to alex alex97
    TI__Genius 10885 points
    Hi Alex,

    This sounds like the culprit, as the PMIC requires a valid clock to operate, and a typical crystal should stabilize within 500ms of the first energy insertion.

    The following app note may help with evaluating your current layout for the crystal, but using an external clock would adequately address this behavior as well:

    www.ti.com/.../swca076.pdf