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TPS61200 not fully turning off

mflaga
Prodigy 30 points
Other Parts Discussed in Thread: TPS61200

I have implemented the TPS61200 to regulate a 2xAA to 3.3V, whereas when EN is low (disabled) I see Vout is 2.5V. Not OFF as it supplies enough current to keep things kind of going. It is on my prototype. I was depending on the EN to be driven by button diode OR’ed with a PNP from the micro to clamp to VIN to either enable the power on or release the enable to a pull to GND. Shorted out the EN to GND and still see 2.5V on the Vout. I have gone over the designs and assembly, there is no SHORT between VOUT and VAUX.

To make things simpler I have removed Q1 and making the BUTTON a straight ON/OFF and still have the same symptom. I have also removed R2 so UVLO is tied to VIN, as it too, was causing problems.

  • 0
    Prodigy 30 points

    The schematic did not come across. So i have attached it here.

    1200.mpf_tps61200.pdf

  • 0 in reply to mflaga
    TI__Guru**** 150950 points

    Could you look at the L pin to see if the IC still switches?  If this pin simply shows your input voltage with no switching, then the IC is indeed off.  Something else is causing the 2.5V to appear on the output (another power supply, leakage, etc.)

    Could you make sure and look at the EN pin with a scope right across the IC, from the EN pin to ground?  IR drops might cause the voltage you measure at other points on the board to be different than what the IC actually sees.

  • 0 in reply to Chris Glaser
    Prodigy 30 points

    It turns out it was getting it from elsewhere. I initial efforts of debugging I jumped PS to Vin and turned the IO driving the PS to Input, rather than output. This back washed through the Micro's "Input" and presented itself as +2V on VCC or Vout. The L was indeed stopped switching. Removing the L showed the +2V still there. And removing the TPS61200 still showed the +2V. Hence I then found the jump.

    The jump was initial attempt. But the problem preventing the start up, turned out to be that I need to have Voltage divider on UVLO, much much higher than the data sheets 250mv+/-15mv. It does not seem right. but it works.

    It now starts and stops as designed.

    Thanks.