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TPS24740EVM-596: waveform questions

Miranda Gu
Genius 16632 points
Part Number: TPS24740EVM-596

Hello team,

My customer has two questions during TPS24740EVM test.

Test condition:

Input: VINA (VPSU): 12.2V;  VINB (VBBU): 11.5V

Load: 2.5A

open J2 and J4

  1. When VINA powered on (without VINB), why the waveforms have a drop?

       2. When disconnect VINA, why VINA still has voltage? 

Appreciate your support!

  • 0
    TI__Guru 57737 points

    Hi Miranda,

    1.  When you say, 'VINA powered on (without VINB), , does that mean you are applying voltage only to VINA and there is no voltage applied to VINB ?

    Did you capture the input current waveform ? Can you add a electrolytic capacitor, say 1mF at the input. This is to check if the voltage drop reduces. The voltage drop can happen if there is inductance in the path of power supply and the EVM. Using a local cap at input should help.

    2. Do you mean when VINA is disconnected why there is still voltage on VBBU ? Can you clarify what are the voltage captured in the image that you have shared ? 

  • 0 in reply to Praveen GD
    TI__Genius 16632 points

    Hi Praveen,

    Sorry my words make you confused. Let me clarify:

    For 1st waveform, I want to ask when apply voltage only to VINA (VPSU), no voltage applied to VINB (VBBU), why there is a droop in VINA & Vout waveforms?

    For 2nd waveform, I want to ask when apply voltage to both VINA (VPSU) and VINB (VBBU), then disconnect VINA, why there is still voltage on VINA pin? looks like the voltage is from VINB.

    Appreciate your kind support!

    Miranda

  • 0 in reply to Miranda Gu
    TI__Guru 57737 points

    HI Miranda,

    For the 1st waveform, as I mentioned the drop in Vin and Vout could be due to source inductance. To reduce this you can add a local capacitance at the input of the VINA on the EVM. This should reduce the drop in voltage when there is a sudden change in current flowing into the load.

    For 2nd waveform, Can you try discharging the voltage on VINA using a resistor. VINA turns OFF when its voltage drops to below VINB and reverse current flows in the VINA channel. The voltage you measure on VINA could probably be some residual voltage and it should drop to zero if discharged using a resitor.

  • 0 in reply to Praveen GD
    TI__Guru 57737 points

    Hi Miranda,

    Were you able to test the circuit as I detailed in my previous message ?

    Do let me know if you have further questions ?

  • 0 in reply to Praveen GD
    TI__Genius 16632 points

    Hello Praveen,

    For 2nd waveform, customer use 1A load to discharge the residual voltage, took 10~20 seconds to discharge VINA (PSU output) to zero. When using 6A load, the VINA (PSU output) immediately discharge to zero. I believe that is because customer's PSU has large output capacitance and stores much electrical charge.

    Customer want a solution to discharge the VINA residual voltage, do you have any suggestions?

  • 0 in reply to Miranda Gu
    TI__Guru 57737 points

    Hello Miranda,

    Unfortunately, we do not have any integrated solution which can discharge input. This is because it is challenging to define when the input supply is disconnected if there is huge capacitance on the input and no load at the output. 

    May be you can try some discrete solution like a resistor in series with a signal FET to GND and turn ON the signal FET when input power supply is OFF.