• State TI Thinks Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 395 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.

LM25085: Can I turn off the PFET during shutdown?

Cameron Hosken
Prodigy 30 points
Part Number: LM25085

I am using an LM25085 to regulate a Lion battery stack. When fully charged the stack is at about 32.5V. Discharged is in the mid 20's somewhere. My VOUT set point is 29V. I'm doing this to protect legacy equipment for which the supply must not exceed 30V.

This equipment also has a secondary power input for a 28V bench top supply when not running on batteries. This secondary supply is connected to the VOUT point of the LM25085 circuit.

My goal was to put the LM25085 into shutdown mode by grounding the RT pin when the bench top power supply is connected. I was expecting the PFET to turn off and block current flow back into the VIN section and protect the batteries. However when I pull RT low, it appears that the PFET gate is pulled to VCC which leaves the PFET conducting. I could add a diode to the power path but there are several reasons I don't want to do this, lack of board space among them.

My question is this: Is there a way to put the LM25085 into shutdown mode with the PFET off?

Thanks,

Cam

  • 0
    TI__Mastermind 40060 points
    Hello Camreron,

    According to the Datasheet the VCC regulator is disabled and the PFET should be off. Can you retry without the secondary supply connected to the output?

    Also you may want to try a 10k from Gate to source of the FET to discharge the Qg of the PFET?

    Hope this helps?

    David.
  • 0 in reply to David Baba
    Prodigy 30 points

    Yes, I read that too which is why it surprised me that I measured the VCC voltage at the gate.

    The 10k from gate to source is an interesting idea.

    I'll try your suggestions and report back.


    Thanks,

    Cam

  • 0 in reply to Cameron Hosken
    Prodigy 30 points
    I re-took measurements of the gate voltage and you are correct that when I ground RT the line pulls high. I must have had my cursor measuring the wrong channel of my scope or something. User error as usual.

    The 10k resistor from gate to source did nothing.

    Upon further investigation and research it looks like the body diode in the fet is conducting in the reverse direction. This is intrinsic to basically all mosfets and why I see the voltage I've applied to VOUT at VIN less some drop.

    So to implement the functionality I want I have to either use back-to-back fets or a diode in series with the fet.

    Thanks,
    Cam
  • 0 in reply to Cameron Hosken
    TI__Mastermind 40060 points

    Hello Cam,

    I do apologize, I did not realize Vout>Vin. under your test conditions.  If this is the case, you will need to place a diode in series with Vin to block reverse current.

    Regards,

    David.