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Original question:

PMP4448: Design suggestion to use TPS92314

PMP4448: Long turn-on delay

Ronald Reichmuth
Prodigy 30 points
Part Number: PMP4448
Other Parts Discussed in Thread: TPS92314

Dear Support Team

Today I was testing our first Design with TPS92314. Our design is more or less a copy of PMP4448.

Now I'm trying to optimzie the turn-on delay (A long start-up time gives a sensor light the impression of a reduced detection range.)

What is the optimum/limits for capacitor size at VCC (PMP4448: C4 low as possible) and pull-up to Vin (PMP4448: R6 and R7, high as possible). Or are there any other ways to start the TPS faster?

Our input voltage requirements are 100 to 240VAC, and it seems only that only the startup delay at 100VAC is too long (>2s) and should be < 500ms.

Thank you for your support.

  • 0
    TI__Intellectual 965 points

    Hi, Ronald,

    For the VCC capacitance, there is indeed limits because VCC capacitance will provide the internal circuit power at the startup before transformer auxiliary winding take over.

    10uF is good start also verified in TPS92314EVM. Also we can reduce the R6, R7 to improve the startup time sacrificing the standby power(also checking the R6,R7 resistor power rating when reducing the value). On the TPS92314 EVM, with 2*75kohm startup resistor and 10uF, there is around 390ms turn on delay.

    Thanks.

  • 0 in reply to David Ji
    Prodigy 30 points

    HI David,

    thank you for your feedback and improvements suggestions. I tried tu reduce the resistor value to 2*75kOhm. But as I understand there is an issue with the startup at 100VAC (please have a look to the attached pictures) there is some 'hiccup' at the startupt. Any idea where that comes from?

     

  • 0 in reply to Ronald Reichmuth
    TI__Intellectual 965 points

    Hi, Ronald,

    It seems that the auxiliary winding power supply not provide power for IC before output voltage stable owing to VCC drop quickly. I have got the PMP4448 on hand and will feedback results to you tomorrow using 2*75kohn resistor as startup resistor.

  • 0 in reply to Ronald Reichmuth
    TI__Intellectual 965 points

    Hi, Ronald,

    Thanks for providing the waveform. Today, I have tested PMP4448 and replicate your issue.

    From the 100VAC, owing to slow charge output, VCC also drop down its turn off threshold. 

    The following recommendation is:

    1. increasing the VCC cap to 22uF or 33uF and decrease the startup resistor(around 10kohm) to as smaller as possible. At the same time, there is need to care the power loss on startup resistor especially at high line.

    2. decreasing the Buck output capacitor, it will also help to improve the turn on delay.

    Thanks.