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TPS40428: Power up and Power down

Srinivas Jagarapu1
Prodigy 130 points
Part Number: TPS40428

Hello,

Please see below snip from TPS40428 datasheet. We could able to meet the power-up requirement.

Is the power-down requirement is equally critical? Please see our design power-down.

Yellow VDD (5V) of the power stage and green PWM signal.

Regards,

Srinivas

  • 0
    TI__Expert 3525 points

    Hi Srinivas,

    Yes, the power down is also important, and the VDD should be maintained until PWM switching operation is completed.

    The issue in the waveform you showed is that, the power stage was still switching ON/OFF when VDD is sagging. While the VDD is supplying the voltage for FETs gate driver, then there is a period the FETs may operate in linear mode which can generate high power loss and may cause potential risk or even damage to the device.

  • 0 in reply to Qingquan Tang
    Prodigy 130 points

    Hi Qingquan Tang,

    Thanks for the reply.

    Please see below Vin, +12V(Green) and VDD, +5V(Yellow) power down in our custom board. 

     

    I am going though SLVUA86, under section 6.4 there it is mentioned about power on and power off sequence.

    In my design there is no way to turn-on/off PWM. As Vin, +12V applied +5V (VDD)  will be generated and I am delaying TPS40428 enabling by adding a cap at CNTL1 pin. So that I can guarantee no PWM signal before power stage VDD is stable.

    Power down: Vin (+12V) goes off first followed by VDD(+5V). I don't have a control to turn off PWM.  

    In the board where the FET got damaged, didn't meet both power up and power down sequence, i.e. PWM signal was available before the VDD reached 4.5V. So, I am not sure whether the power-up or power-down caused the damage.

    Thanks,

    Srinivas

  • +1 in reply to Srinivas Jagarapu1
    TI__Expert 3525 points

    Hi Srinivas,

    For the power down, can you add more delay between 12V and VDD? As you may see that VDD is sagging when Vin was about 5~6V. It's improved but still the FET can generate extra power loss and heat during the VIN dropping period. It's better to wait for VIN to be fully OFF first before VDD drops.

    Improper power up/down sequence may not cause damage every cycle but there is potential risk to damages. It's strongly recommended to follow the suggestions in datasheet to prevent issues during power cycle.