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TPS62736: device is passing VIN to VOUT ... why?

Timothy Sweeney-Fanelli
Prodigy 10 points
Part Number: TPS62736

I have a PCB which uses a TPS62736. VIN is connected to the battery (3.7v nominal). Desired output is 3.3V. I use R1=4.12m, R2=634k, R3=8.25m.

In my first run of PCBs -- there was an error on my BOM, and R1 was mistakenly placed as 8.25m. In this version of the PCB, VOUT is 2334mV and the TPS62736 works as expected. I then ordered a corrected set of PCBs with R1=4.12m. In this new set, VOUT is always equal to VIN, and I can not figure out why.

Schematic attached.

Anyone able to help?

  • +1
    TI__Guru**** 151395 points

    Hi Timothy,

     The buck converter turns on as soon as its input exceeds 1.95V typical.  If VOUT is set above 1.95V then the converter runs in 100% duty mode and sends VIN to VOUT until VIN > VOUT setting by at least 200mV.  If you want the converter to wait until its input voltage is higher than the output voltage to turn on, you can connect VIN_OK to EN2.   

    The resistor values look correct for VOUT=3.3V and VIN_OK=3.819V. However, high value resistors are sensitive to parasitic resistances formed by residual solder flux. I suggest removing the resistors, cleaning any solder flux that could be forming parasitic resistance across those resistors and replacing them on their side to reduce surface area for contact.  Or you can replace the resistors with new resistors having 1/10 to 1/100 of the current values.  This will eliminate parasitic resistance as the issue.  You could also look at the VIN_OK output pin and see if it is tripping at 3.819V

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Jeff,

    Thank you... I'm curious about tying VIN_OK to EN2 ... I understood VIN_OK to be a logic level output, not a voltage level input? I am OK with the buck turning on past 1.95V typ, but would actually prefer it not turn on until after 3.3 (specifically, to cut off below 3.3). 

    I will replace the resistors on the board with:

    R1 = 41.2k (R11 on my schematic)
    R2 = 6.34k (R12 on my schematic)
    R3 = 82.5k (R13 on my schematic)

    to eliminate the parasitic effect as you suggest ... but it doesn't explain why my first run on PCBs (which the incorrect, higher-valued R1) work fine and the second round doesn't. Do I just chalk it up to variance in manufacturing, and expect that eliminating the parasitic effect will resolve?

  • 0 in reply to Timothy Sweeney-Fanelli
    TI__Guru**** 151395 points

    Regarding VIN_OK, high output is referenced to VIN, the same as EN2 per below:

    Regarding the parasitic effect, it is possible that one run of boards was good but another is not.  I had this problem with our EVM vendor so we switched to a vendor with a before board cleaning machine.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Very embarrassingly - have found the root cause here: an error in the BOM. R1 (R11 on my schematic) was placed as 4.12 Ohm, not 4.12 MOhm; which puts VOut at an impossible 16.995V.