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Original question:

TPS40210 can not startup with large output capacitance

TPS40210-EP: Restart issues with large SS capacitor

Lorenzo Lutti
Intellectual 655 points
Part Number: TPS40210-EP
Other Parts Discussed in Thread: TPS40210

Hello, it looks like my circuit using a TPS40210 (I'm using the EP version, if that matters) has some issues restarting from temporary power interruptions when a large soft start capacitor is used. I have a very big capacitive load at the converter output, therefore if I use "typical" Css values (1 uF or less) the hiccup overcurrent protection trips and the conversion does not start. I've fixed this by using a bigger soft-start capacitor (10 uF), but with this configuration another problem appears: if I disconnect the input voltage for a short time (let's say less than 1 second), the converter "shuts down" forever until the next power cycle. The output voltage slowly drops to the input voltage, and I don't see any activity on the GDRV pin or the switching node. Instead, for longer power interruptions (let's say 2 seconds or more) the conversion restarts normally: the output voltage ramps up to the final value.

I suspect that the short power interruptions "upset" in some way the soft start and overcurrent stage of the controller (perhaps some sequential logic goes into an invalid state?).At the moment I've fixed this issue by connecting a 10 uF capacitor on the BP pin. I haven't done extensive tests yet, but with this modification the converter seems to start always, independently from the power interruption duration.

My questions are: is this a known issue? Is increasing the Cbp capacitance a suitable/preferred solution to fix it? The datasheet clearly states to use a 1 uF value for Cbp.

Thank you!

L.

  • 0
    TI__Guru 58215 points

    Hello Larry,

    I expect that the overcurrent protection trips in this case. Would you please check the current signal on the sense resistor?

    Best regards,
    Brigitte

  • 0
    Intellectual 655 points

    Hello Brigitte, sorry for the big delay. Unfortunately I cannot work on the prototype at the moment: as soon as it worked, they snatched it from my hands.:)

    It is likely that the overcurrent protection trips, but then after a certain time (depending on equations 4, 5, 6 on the datasheet) the IC should try a re-start. Instead, it just shuts down until the next power cycle.

    I suspect that if during the power interruption the voltage on the BP pin drops to a certain level, for example between VSS(ofst) and VSS(rst), when the power is re-applied some internal combinatorial logic goes into an invalid state. Maybe increasing the value of the BP capacitor prevents this.

    L.

  • 0 in reply to Lorenzo Lutti
    TI__Guru 58215 points

    Hello Larry,

    How long did you wait for the device to restart? If the OCP trips, the SS cap is very slowly discharged and therefore it takes extremely long until the device tries to restart.

    The only possibility to check out if OCP trips is by measuring the voltage on the sense resistor. I am sorry, but without this measurement we will never know if this is the root cause in your case.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    TI__Guru 58215 points

    Hello Larry,

    As I did not hear back from you, I expect that you have been able to solve this issue. If this is not the case, please post below or create a new thread if this one is already locked.

    Best regards,
    Brigitte