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TPS63901: Soft start operation

Dice-K
Mastermind 7470 points
Part Number: TPS63901

Tool/software:

Hi support team,
Currently being evaluated with TPS63901EVM.

I checked the operation during soft-start and found something that concerned me.

TPS63901_Soft-Start_250509.xlsx
It is set to increase from 1.8V to the set value (3.9V in this case) in 100mV steps, but I found that the input current increases and the input voltage drops with each step.
I have a few questions about this operation.

[Question]
1. Please let me know what causes the input current to increase. I would like to understand what is happening inside the TPS63901.
2. Please let me know how to reduce the input voltage drop.
3. I have confirmed that setting the Input Current Limit reduces the input voltage drop, but is this the correct way to do it?
4. Is it possible to change the Input Current Limit after the output voltage has been set?
For example, I could prepare two resistors for the Input Current Limit setting and switch between them.

Regards,
Dice-K

  • +1
    TI__Mastermind 32350 points

    Hi Dice,

    Nice to hear you again.

    Comments for your questions as below, please check.

    1. Please let me know what causes the input current to increase. I would like to understand what is happening inside the TPS63901.

    A: If my understanding is correcy, you mean the peak input current is higher when Vout is over 1.8V threashold, right?  it is about soft-start logic of TPS63901, before 1.8Vout, device start with 500mA peak inductor current limit, after that, device rising Vout with 100mV each step and current limit is nominal one. The actual peak current after 1.8Vout is very depend on your effective Cout.


    2. Please let me know how to reduce the input voltage drop.

    A: Voltage drop at input is caused by oscillation between ESL of your input cable and Cin, shorter your cable should be help. Also, increase your input cap and decrease your output cap should help too.


    3. I have confirmed that setting the Input Current Limit reduces the input voltage drop, but is this the correct way to do it?

    A: Do not recoomend do this to reduce Vin drop, because as you mentioned, Vout may crashed if there is a higher load.


    4. Is it possible to change the Input Current Limit after the output voltage has been set?

    A: unfortunatly, you can not do this. Because we will close R2D function after start-up phase done to save power.

    Regards

    Tao