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Original question:

TPS92643-Q1: CSP-CSN

LM51551-Q1: LM51551 problem with long startup time

zhang lin
Prodigy 200 points
Part Number: LM51551-Q1
Other Parts Discussed in Thread: LM51551

Tool/software:

We are using LM51551-Q1 chip, Topology: BOOST, output voltage: 46V;

The picture is the schematic diagram and the waveform of startup time. The first picture VOUT output voltage is jagged, and the start-up time is very long 931ms;

The yellow waveform is the VOUT output voltage waveform；The red waveform is the EN control voltage waveform;Is this waveform normal? What causes the long startup time?

The second picture is the test result of changing R383 resistor from 1K to 0, and the startup time becomes 277ms. We suspect that there is a problem near RCS resistor, please help us look at our circuit diagram, explain why there is a jagged output voltage; At the same time, help to see how to modify the parameters to shorten the startup time;

  • 0
    Prodigy 200 points

    vout=46V IOUT=1A

  • +1
    TI__Mastermind 21810 points

    Hi Zhang,

    Thanks for using the e2e forum.
    LM51551 has an overcurrent hiccup mode feature. If the device detects overcurrent for several cycles, it will disable the device for a longer period of time and then resets.
    This creates this hiccup behavior you are seeing and explains why the time is related to the circuit at CS pin.

    Instead of modifying the current sense circuit, I would recommend to choose a longer softstart time.
    Right now, a softstart cap of 10nF is used (C717). Increasing this capacitor slows down the ramp up of the output voltage, therefore reduces inrush current and avoids triggering the overcurrent limit.

    Please let me know if further help is needed.

    Best regards,
    Niklas