• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 555 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS92691: Buck Design Refusing to "Turn On" After Off Periods Using PWM

Joshua Evans77
Prodigy 20 points
Part Number: TPS92691

I have a buck design for this LED driver (I've used it as a boost in the past), and I'm at my wit's end on what is happening with this guy. Reference my schematic below. The system seems to work beautifully, if, shortly after providing power on BATT_PWR, I bring the PWM signal high (my input logic is active low per schematic). I have left it running for 8+ hours without issue. The problem I run into is the way I plan on using the circuit, is to "power off" using the PWM input. When I turn the system off with PWM (keeping in mind there is still power to BATT_PWR), and leave it off for more than a few seconds, it refuses to continue switching or turn back on until I do a full power cycle on BATT_PWR.

During this time it's refusing to turn on back on, I've checked health on VCC, CSP/CSN, IMON, IADJ, PWM, etc. and everything looks great. Is there a timeout feature on this chip that I'm missing? Something that maybe puts it into a fault condition after a certain amount of time? In the meantime, if anyone sees anything wrong with my design, please let me know, it would be much appreciated! I'm still digging through the datasheet looking for a clue but it's stumping me at the moment.

  • 0
    TI__Mastermind 18890 points

    Hi Joshua,

    The right way to disable the part is to pull SS to ground with an open drain or collector circuit.  When you pull PWM down to ground the part is still active but it turns off the PWM FET (Q2) while tri-stating the COMP pin and disable the gate drive to the main power FET (Q3).  While PWM is low, the ouput will droop due to leakage and impedance that is tied to the output.  If it's off for a long time then this ouput voltage will discharge completely.  When PWM goes high again the part will not have any Soft-Start feature and will get an over-current event.

    Again, the right way to disable the part is to pull SS to ground with an open drain or collector circuit.  This is described at the bottom of section 7.1 in the data sheet.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 20 points

    We have tested this and it does seem to work. One thing to note is that, once in a failed state, this will not recover the IC (based on testing so far). Thank you for the help! I will say, the one mention of this in 7.1 is a few words at the end of a sentence. Being such an important feature, the datasheet should be revised to call more attention to this.

  • 0 in reply to Joshua Evans77
    TI__Mastermind 18890 points
    Hi Joshua,

    Good to know that you were able to get it to work.

    Thnaks Tuan