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TPS61175: Slow Start pin discharge issue

Martin B.
Prodigy 30 points
Part Number: TPS61175
Other Parts Discussed in Thread: TPS25200

Hello,

I noticed that despite that in the datasheet it is specified that the soft start pin is getting discharged with 5k if the device is disabled, I did not observe this on our circuit. The SS capacitor is quite large in our case (4u7), but the capacitor should be well discharged after about 50ms (+10ms EN latency). However, it discharges much slower in our case, so we need to add some external parts (a Schottky diode to EN with a 10k pull-down helps) to discharge it during a reset event.

Is this a known issue of this device?

Thank you for your answer!

  • 0
    TI__Mastermind 34201 points

    Hi Martin,

    4.7uF for SS capacitor is quite larger. The RC constant is 4.7u*5k=23.5ms. To fully discharged, it needs 23.5ms*5=117ms. 

  • 0 in reply to Zack Liu
    Prodigy 30 points

    Hi Zack,

    thanks for your reply. The RC constant was much longer in reality, though, that's why I was skeptical since it does not seem to be 5kOhm.

    I found an post from 2014 on that which could explain it in parts, however, I observed it slightly differently:

    > If you use a large ss cap (larger than 1uF), there is a small design bug in the circuit. If EN is low, then the ss pin voltage will not discharge to 0 and will remains at 0.5V. It will lead to large inrush current when you start up again.

    e2e.ti.com/.../1258826

    It would be great if these things appear on the errata sheet, since that is very helpful information.

    kind regards,
    Martin

  • 0 in reply to Martin B.
    TI__Mastermind 34201 points

    Hi Martin,

    Thank you sharing this post. 

    May I know the discharge time in your result?

  • 0 in reply to Zack Liu
    Prodigy 30 points

    Hi Zack,

    I digged a bit more into the issue and I found that indeed the discharge time is as expected with 5k and the problem is more like the issue in the linked post. The SS discharge stops after 20ms at about 0.9V...1.0V as you can see in the oscillograph:

    It then only discharges very slowly, so that on the next enable after a few seconds, the current will be too high, an external TPS25200 current limiter trips (dip in VIN) and it tries to start again, while SS gets a bit discharged. It can then start properly as SS is less than a critical level that will not trip the current limiter.

    With a schottky diode to EN, SS can get discharged to <0.3V and it can then be properly enabled later.

    Regards,
    Martin