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tps61070 unstable regulation

Boris Snajder
Prodigy 60 points
Other Parts Discussed in Thread: TPS61070, TPS61071

I'm using TPS61070 to 3.3V power to a device from alkaline batteries. Problem that I have with TPS61070 is that it shows unstable regulation (output voltage varies from 2V to 3.8V) when two batteries are connected in series (input voltage somewhat higher then 2.5V). If only one battery is connected (input voltage below 1.5V) there is no problem with regulation, and output voltage is stable at 3.3V.

Input and output capacitors are 10uF, an inductor is 4.7uH. Does anybody have any idea why I'm seeing this behavior?

Thanks

  • 0
    TI__Guru**** 191445 points

    Can you post some waveforms showing both behaviors?  Include the SW  node.  What is your load?

  • 0 in reply to JohnTucker
    Prodigy 60 points

    Hi,

    I've attached waveforms. You can find output voltages and inductor voltages for steady-state and startup for input voltage 1.5V and 2.5V. What I've noticed is that when I connect probes, output voltage is stable in both case. So, there is some problem with compensation in control loop (my voltage divider is 180k and 1M). However, there is some transient spike during startup when input voltage is 2.5V (you can observe that in waveforms). As far as I can see, TPS61070 changes duty cycle during startup and that is reason why I have different voltages, just I do not why.

    Thank you.

    tps61070_waveform.zip
  • 0 in reply to Boris Snajder
    TI__Guru**** 191445 points

    looks like you may be operating in skip mode.  You can try to increase your load a little to see if it goes into CCM mode.  Or you could try a TPS61071 with skip mode disabled.

  • 0 in reply to JohnTucker
    Prodigy 60 points

    Yes, it seems that I'm in skip mode. However, my current load is about 25mA, but when deployed my device should be the biggest part of time in sleep mode requering only few uA. Thus, this skip mode would enable to my device to have longer battery life.

    So, you think that this power saving mode by entering skip mode could be trigger of unstable regulation? And that regulator without that feature shouldn't be unstable? Unfortunatly, I do not have TPS61071, so I can not try that, but I'll give a try to force TPS61070 to go into CCM mode and check if I still have overshoots during startup.

  • 0 in reply to Boris Snajder
    TI__Guru**** 191445 points

    It's not technically unstable, you are just seeing short bursts of switching followed by periods of sleep.  It is normal for the ripple to increase during power saving mode.  I am somewhat surprise that you are seeing that much ripple though, other devices that I normally work with have less. 

  • 0 in reply to JohnTucker
    Prodigy 60 points

    Yes, skip mode is not instability. However, I would tell that probes capacitance made it stable (to some degree). As, when I was watching output voltage only with mulimeter output voltage was going from 2.0V to 3.9V, and then back down again. While probe from scope was connected to the output voltage, voltage was stable at 3.3V. Just, I can not explain why during startup there is overshoot to 3.5V.