TPS24751: Use of R/C on output for high In-rush current load.

Part Number: TPS24751

I am trying to power a large LCD 12V/8A(nom) LCD display from the output of the TPS24751 circuit.

The LCD can have as much as 26A in-rush current when the backlight powers up.

This is over an approximate 3ms period.

Would it be prudent/advisable to change/set the Dv/Dt rise-time on the output by adding the RC to the Gate control output pin of the TPS24751?

If so, what values would you recommend?

Thanks in advance.

8 Replies

  • Hi Clayton,

    Thanks for reaching out!

    The attached app note explains about the selection of Cgate to control inrush current. Please go through it and revert back for any further queries.

    slua740.pdf

    Best Regards,

    Rakesh

    --------------------------------------------------------------------------------------------------------

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  • In reply to Rakesh:

    Hi Rakesh,
    Sorry for the delay in replying.
    I have now followed the suggested details in the app note, and have it working.....nearly :)

    The issue I now have is that, if the load is connected to the output, and I then enable the eFUSE (EN = high, > 1.35V), the eFUSE shuts down (goes into hiccup mode). However, if I leave the load disconnected, enable the eFUSE, and then connected the load it's fine!!!
    I have tried various in-rush values, but this does not seem to solve the problem.

    The load draws an idle current of 7A but has an in-rush current up to 26A for 3ms.

    Any suggestions?

    Thanks.

  • In reply to Clayton Whitehead:

    Hi Clayton,

    Please fill-up the design calculator www.ti.com/.../toolssoftware and send me for review. Include all the load capacitance and expected start-up load resistance.

    Can you send schematic and below test waveforms
    1. Vin, Vout, GATE, Timer
    2. Vout, GATE, Timer, input current

    Best Regards,
    Rakesh

    --------------------------------------------------------------------------------------------------------

    To find the latest information on Power Switches, Visit www.ti.com/powerswitch 

  • In reply to Rakesh:

    Hi Rakesh,

    Thanks for the quick response.

    I will get the test waveforms for you.

    In the meantime, just to let you know, the schematic is the same as for the datasheet except for the following:-

    1) We are using a TTL signal on the EN input (having isolated it from the OV circuit and re-configuring the resistors for 5V instead of 12V.
    2) We are using the Blocking FET as per Fig. 46 in the datasheet.

    Output capacitance is 470uF with diode across it (same as for Demo board)

    One question, when the datasheet refers to output capacitance (Cout), are we talking only of the eFUSE circuits output capacitance (470uF in this case), or do we need to include any load capacitance as well? Unfortunately, the datasheet does not appear to make this clear.
    If this is the case (need for including load capacitance), what is the best way of ascertaining this, as the load I am driving is a large Frame 12V/8A monitor and the datasheet for it does not give this information?

    Thanks for your help with this.

    6888.TPS24751_Design_Filled.xls

  • In reply to Clayton Whitehead:

    Hi Clayton,

    The device fails to startup a combination of load capacitance and heavy load. As you see, in datasheet example Figure 41, the Rload is enabled after device asserts PG signal on successful startup. This datasheet example showcase the inrush current control while driving 470uF capacitance.
    Also note, eFuse does not required to have any load capacitance for operation.
    If your system needs to start with heavy load, I suggest to remove load capacitance 470uF and check.

    Best Regards,
    Rakesh

    --------------------------------------------------------------------------------------------------------

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  • In reply to Rakesh:

    Hi Rakesh,

    Thanks for the suggestion.
    I have removed the 470uF on the immediate output of the eFUSE circuit, but this still (on it's own) did not solve the problem.
    I then tried increasing the Power Limit for the internal FET by reducing the Rprog value down to minimum (4.99K).
    This seems to do the job. The output will now switch the display on and off, when always connected, when the EN line is toggled.
    I will continue with some more tests and let you know how I get on.

    Another question with regards the Rprog resistor value.
    The datasheet states setting it to 4k99 to allow maximum power limit. Can I reduce this to a 4k7 to use a standard resistor (5K1 doesn't work)?

    Thanks again for the assistance.
  • In reply to Clayton Whitehead:

    Hi Clayton,

    It is not advised to keep power limit beyond 30W.

    Can you send the schematic and below test waveforms to look which parameter can be tweaked to pass the design.
    1. Vin, Vout, GATE, Timer
    2. Vout, GATE, Timer, input current
    Capture the waveforms for two cases with and without 470uF.

    Best Regards,
    Rakesh

    --------------------------------------------------------------------------------------------------------

    To find the latest information on Power Switches, Visit www.ti.com/powerswitch 

  • In reply to Rakesh:

    Hi Rakesh,

    OK, I'll increase the Rprog resistor value to, say 6K2.
    I'll try to capture the waveforms for you.

    Regards,
    Clayton