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Original question:

TPS2113A: Output Voltage unstable when input changed

TPS2113A: Output voltage drops when switching from IN2 to IN1

Gus Martinez
Mastermind 25311 points
Part Number: TPS2113A
Other Parts Discussed in Thread: TPS2116, TPS2121

I have the following schematic in my design.

XFMR_OUT (IN2) is always on. HSEC_5V0 (IN1) is turned on and off.

When I turn HSEC_5V0 (IN1) on, sometimes the output of the switch starts to drop, then recovers some time later. Can the output of the switch be going into high-z mode? Any ideas as to why this may be happening and how to fix it?

  • 0
    TI__Mastermind 32650 points

    Hi Gus,

    This is more or less expected behavior. Our devices work in a break-before-make methodology. This means that for a period of time during switchover, no inputs are connected to the output. The device recognizes the switchover request, turns off one channel and then turns on the other channel. The dip you are seeing occurs in this time frame.

    While we do offer devices that have faster switching like TPS2116 and TPS2121, the easiest way to reduce this dip is by adding more output capacitance which supplies the load and maintains the voltage on the output rail.

  • 0 in reply to Shreyas Dmello
    TI__Mastermind 25311 points

    Hi Shreyas,

    Per data sheet, the transition time for IN1 to IN2 is max 60us. Is that the right spec to look at? I'm seeing ~2ms in the waveform I attached before OUT starts ramping to IN1 voltage level. 

    Oddly enough this doesn't happen all the time. The majority of the time, the switch happens as I would expect. You can see an example below.

    Just to give more information, IN1 is coming from a toggle switch. I'm wondering if glitching during the mechanical operation of the switch could be causing a problem? 

  • 0 in reply to Gus Martinez
    TI__Mastermind 32650 points

    Hi Gus,

    From the first waveform, the switching behavior of the mechanical switch does not always show this glitching but it is a possibility. Asyou can see with your second waveform, CH2 and CH1 does cross one another due to the switching. With a mechanical switch, the bouncing or glitching could delay the switching further. This could happen in a case where due to the glitching, the device keeps switching back and forth between the two inputs until the switch input stabilizes and the device recovers. I think you may need some output capacitance to hold your output rail and some input capacitance to filter out the input rail. 

    Could you add this capacitance and retest?

  • 0 in reply to Shreyas Dmello
    TI__Mastermind 25311 points

    I added a 10uF cap to the IN1 input and this has solved the problem. It does appear that the toggle switch rapidly connects/disconnects the source when it is flipped and this somehow causes the TPS2113A turns off its output (high-z). It then takes some time for the TPS2113A to re-enable its output. I don't think this is the normal break-before-make operation. In any case, the input cap shields the TPS2113A from this rapid connect/disconnect which solves the problem. Marking this as resolved.