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TPS61202 Startup sensitivity

Geoff Emmer
Prodigy 20 points
Other Parts Discussed in Thread: TPS61202, TPS61252, TPS61200

Hello,

I'm using the TPS61202 to step up a LiPo battery to 5V, with resistors thrown in so the UVLO will trigger at 2.7V. The circuit worked reliably the first couple of power cycles, but now it does not work immediately on startup. I tried probing the UVLO pin to check what voltage was arriving, and that action apparently caused the component to start working again. Further experimentation showed that tapping the junction with an unconnected conductor, or even my finger, was enough to cause the component to start delivering the expected 5V.

Looking on the board, I saw the recommendation that I use the TPS6107x family extended soft-start application note (linked again here) to limit inrush current. I don't need a particularly fast startup time for this; I used a 10uF cap and a 1K resistor, which I know is outside the table provided in the notes. This did not address the problem, but it's possible that my choice of values here is to blame for that.

I've done what I can to rule out bad solder joints. I suspected that I had some physical problem with the board - a component lifting from the board that by pushing with the probe was coming back into contact - but it seems that is not the case, since I cannot "start" it by probing it with a nonconductor, but I can by probing with a conductor. My other thought now is that I have some unintentional fuzziness in my ground, and my probing is siphoning static charge or something. Accordingly, I've attached a schematic showing the conductor on the PCB. The soft-startup components are not shown.

Is there some fault in my grounding that jumps out at you? Maybe there's an aberrant capacitor value in here, although I've checked the datasheet several times looking for that now.

Thank you for your help.

  • 0
    TI__Mastermind 46905 points

    hi Geoff

    sorry for the late reply because of the holiday. hope you have solve the problem.

    since you are using fixed version device TPS61202, The FB pin should connect to Vout directly. and i would suggest place the output capacitor closed to the Vout pin, avoid the via hole in this loop. 

    by the way. why did you choose TPS61202. this device features very low input voltage (0.3V), which is useless in your application. maybe you could have a look at TPS61252, simpler and higher efficiency.

    thanks

    jasper li

  • 0 in reply to Jasper Li
    Prodigy 20 points

    This is embarassing: I wrote the wrong component number in my notebook, and copied that out to the message heading. The component that I meant to use, and have installed on the board, is actually the TPS61200, so the resistors attached to the FB pin should still be appropriate  (per page 15 of the datasheet).

    The TPS61252 that you linked looks good in several ways, but we were looking for the UVLO feature. Since we're using this with rechargeable batteries, we wanted to not drain the batteries below 2.7V, which the settable UVLO feature on the TPS61200 allowed us to do. You're right that the 0.3-2.7V to 5V conversion is a wasted feature for us in this setting.

    To be clear; your recommendation is to remove the via step in connecting the VOUT and FB pins, to address the problem of the device not functioning until a conductor probes the Vout resistor pair junction?