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TPS65105: output cannot maintain when powering up

Gene Qiu
Intellectual 1720 points
Part Number: TPS65105
Other Parts Discussed in Thread: TPS65101, TPS65100

Hi team,

here using TPS65105 in LCD display, 5Vin, 11Vout, Io-400mA.  But found that at first few time powering up, the chip  output rise but cannot maintain 11V out, drops quickly to around 4V.  after powering up 3-4 times, then the output can fully ramp up and maintain 11V as designed,

Changed the output capacitor C4 from 22uF to 220uF or 2*47uF, the issue is solved. Here want to ask what is the reason why the chip output can not maintain, but can be solved by increasing output capacitor.

after change the output capacitor C4 from 22uF to 220uF, the output waveform see below

 

  • 0
    TI__Mastermind 18550 points

    Hi Gene,

    It appears that the boost is getting out of regulation and for TPS65105, the design is such the whole device shuts down if any of the rails is out of regulation.

    Likely cause of the boost getting out of regulation is boost switch hitting current limit. Minimum boost switch current limit for TPS65105 is 0.96A and with 5/11V conversion ratio and 400mA load, the application is likely hitting boost switch current limit. The reason that it can work with higher capacitance at the output could be that once the higher boost capacitance is fully charged, it may be able to briefly supply any additional loads brought on by negative and positive charge pumps coming up. I would suggest to use TPS65100 or TPS65101 which have a higher boost switch current limit.

    Kind Regards,

    Liaqat

     

  • 0 in reply to Liaqat Khan
    TI__Intellectual 1720 points

    Hi Liaqat

    Appreciate for your reply. 

    I checked the inductor, and add one more 4.7uH in series, but now the Vo1 can only reaches to 9.4V below the short circuit protection point(91.25% undervoltage).

    One more observation from myside, when increasing to 220uF output capacitor, the output Vo1 still can exceed the overvoltage(105%) and lasts 370us to 12.5V, then go down. Here i want to check what is the repsonse time when facing overvoltage and short-voltage?

    Any other hints for such abnormal phenonmenon?

    Best Regards

    Gene

  • 0 in reply to Gene Qiu
    TI__Mastermind 18550 points

    Hi Gene,

    Please use the recommended inductor of 4.7uH to allow for enough ripple current for stable operation.

    There is no specification for the time duration of overvoltage/short-circuit in the datasheet so what you are measuring is probably the typical response time.

    Kind Regards,

    Liaqat