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TPS2121: Oscillations/reset in restoring voltage to an input

Part Number: TPS2121

In the circuit above, both sources (IN1 & IN2) are 5V and the output is 5V.

There is a normal source that is DC buck derived (IN2) and the battery (IN1) is to take over when the IN2 disappears.

I have tested in the lab, and I see various results.  I can take either source away, and then on the restoral  of that source, I see one of three results; 1) normal operation, 2) the device will reboot (temporary loss of power),  and  3) an unending oscillation (where the output is going up and down).  When the circuit goes into oscillation, the recovery is to remove both voltage sources and start anew. 

Looking for help on where to look next


  • Hi Ken,

    I am noticing that your SS cap is quite large. This will increase the input settling time and the soft start rise time of the device. 

    When an input becomes valid from an invalid state, the device moves into soft start if that channel is to be used. There is a settling time followed by the soft start rise time and both of these are directly proportional to the size of the SS cap. Refer to Figure 9 on the TPS2121 datasheet for a pictorial representation of this. This is just for your information and is likely why you are not seeing "fast switchover" during turn on.

    I think your major issue is the oscillation. This could be caused because as a input is being loaded, it drops its voltage which then causes the channels to flip back and forth repeatedly. I note that you are no using the ST pin. ST is not only a useful channel indicator, it can also be used to implement hysteresis which aids in solving this oscillation.

    I would recommend you take a look at this TI Design and modify your design to add hysteresis: http://www.ti.com/lit/ug/tidue50/tidue50.pdf

    Another suggestion is that you main supply should be on IN1 while your backup supply should be on IN2. This makes adding hysteresis easier to design.