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Original question:

TPS54335A: PH Pin short to GND issue

TPS54335A: Do not underestimate an UVLO function, either built in to the chip or implemented via EN pin R-divider!

Leo Sheftelevich92
Prodigy 50 points
Part Number: TPS54335A

All,

I am adding this comment because the issues I found in E2E forum is marked as "closed" but there is no a "closing comment" which others could learn from. 

I want to make sure that many people are using this chip successfully. 

The issue's title is "TPS54335A: PH Pin short to GND issue" and it's located in POWER MANAGEMENT FORUM (couldn't include a link here: this website does not handle it well)

I see three bucks discussed there: 8.8Vout, 5Vout, and 3.3Vout. Based on the datasheet formulas, all three bucks are turned on at 5.7V and turned off at 4.8V by EN pin. All 3 bucks have the SAME R-dividers at EN pin: 220k/56k. So, in the 5V and 8.8V bucks, the control loops would try to make the bucks "boosting" when Vin < Vout by staying in 100% duty cycle for long time. This might cause various problems. 

Same applies to some of the 12Vout bucks I've seen in other discussions on E2E website. Do not underestimate an UVLO function, either built in to the chip or implemented via EN pin R-divider!

Leo

 

 

 

 

 

  • 0
    TI__Genius 10070 points
    Hi,

    Although the voltage of turn on is lower than the default output voltage, but the output voltage increases from 0, the duty cycle is not 100%, unless the input voltage increases slowly and the input voltage is still lower than the default output voltage when the soft-start time is over.

    Shawn
  • 0 in reply to Shawn Nie
    Prodigy 50 points
    Hi Shawn,

    Thanks for your comment. I agree, in some cases the 2ms softstart of the TPS54335A could be longer than the VIN risetime, or at least its part that matters here. Still, in many cases the turn-off time and turn-off EN-pin threshold level could be critical, esp. when at low load the turn-off time takes several mseconds. Also, many 24V p-supplies have 10-20ms softstart, that might have 1-2ms/s slewrate. It is much safer using both turn-on and turn-off thresholds at EN pin being above VOUT with a margin (a couple of volts would be reasonable for the turn-off threshold).

    If VIN is applied very fast, such as via hot-plugging or by a relay, the "input filter caused oscillation" might damage the IC.

    Leo
  • 0 in reply to Leo Sheftelevich92
    TI__Genius 10070 points

    Hi Leo,

    You are right, the fast slew rate of input voltage will increase the risk of damage, but this is corner case, our parts can ensure it is save at most of applications.

    Shawn

  • 0 in reply to Shawn Nie
    Prodigy 50 points
    This is why we keep using your parts, Shawn. Of course, the application circuits should be correct, esp. the turn-off threshold setting (per my earlier comment).

    Thanks for your feedback.

    Leo