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TPS61378-Q1: Can't exit Down mode when 0.5A load

Ruixin Feng
Intellectual 2305 points
Part Number: TPS61378-Q1

Tool/software:

Hi team,

when test TPS61378EVM, with 0.5A load, 5V output, when increase Vin from 3.3V to 4.2V, the device enter into down mode (OUT pin is 6.14V). Then customer decrease Vin to 3.63V , but device didn't exit down mode(OUT pin is 6.55V). Could pls help to share some suggestion to solve it? Thanks!
Rayna

  • 0
    TI__Genius 16730 points

    Hi Ruixin,

    Let me check and reply to you tomorrow.

    Regards

    Lei

  • 0
    TI__Genius 16730 points

    Hi Ruixin,

    I verified on TPS61378EVM, with 0.5A load, 5V output, 2.2MHz and mode = H, the device enters into down mode when Vin is above ~4.28V and leaves down mode when Vin is below 4V.

    Could you verify again? And also, please provide the detailed work conditions, such as frequency and work mode.

    Regards

    Lei

     

  • 0 in reply to Lei Zhong
    TI__Intellectual 2305 points

    Hi Lei,

    I checked again. Customer also use 2.2MHz and FPWM, but they changed current limit resistor R5 to 50kohm and found the problem. 

    Thanks!

    Rayna

  • 0 in reply to Ruixin Feng
    TI__Genius 16730 points

    Hi Ruixin,

    Let me check again and provide the feedback tomorrow.

    Regards

    Lei

  • +1 in reply to Lei Zhong
    TI__Genius 16730 points

    Hi Ruixin,

    I changed the R5 to 50kΩ and similar behavior was observed: the device can't leave the down mode in this working condition.

    It's not a bug. Please see my comments below:

    • When R5 = 50kΩ, the current limit is 1.9A. So when Vin = 4V, Vout = 5V and not in down mode, the max Iout is around 1.2A, higher than the Iout (500mA), so the 5V Vout can be regulated.
    • When the Vin rises above ~4.3V and down mode is triggered, the current limit will be around 2/3 of the normal current limit, that is, 2/3* 1.9A = 1.267A. And in the down mode, the ISO FET is partial on and the efficiency will drops significantly. With so, the max Iout also drops significantly.
    • Then when Vin decreases from 4.3V to 4V or lower, the max Iout < 500mA, the Vout can't be regulated and decreases as the Vin. There is no enough gap between the Vout and Vin, that's why the device can't leaves the down mode.

     Increase the current limit (make the R5 smaller) or decrease the Iout (<500mA) can make the device enters and leaves the down mode normally.

    Regards

    Lei

  • 0 in reply to Lei Zhong
    TI__Intellectual 2305 points

    Hi Lei,

    Got it. Many thanks!

    Rayna