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TPS53355EVM-743: I want to know about D-cap control.

user5244340
Prodigy 50 points
Part Number: TPS53355EVM-743

We are considering the output capacitor of the DCDC converter, and we are evaluating Load Transient.

When evaluated using TPS53355-EVM, the results were as follows:

Output capacitor

amount

overshoot

undershoot

MLCC 100uF

5

69mV

40mV

PolymerAL 100uF

5

78mV

42mV

PolymerAL 100uF

4

94mV

42mV

Question 1: Why is the undershoot value smaller than overshoot?
Question 2: By changing the output capacitor, we can see a change in the value of the overshoot.
                   Why is the undershoot almost unchanged?


I think I don't have enough information, so please let me know if you have any additional information.

  • 0
    TI__Genius 12735 points

    Hello,

    The expert for this device is currently out of office and will be returning to office on Monday March 2nd. In the mean time, can you provide a scope shot of the output and the load during the load step? I might be able to provide some insight if I can look at the exact wave forms.

    Best regards,

    Layne J

  • 0 in reply to Layne J
    Prodigy 50 points

    Hello,
    Thank you for your reply.

    I have attached measurement data, would you please check it??

    Best regards,

    Yuya Fujiwara

  • 0 in reply to user5244340
    TI__Genius 12735 points

    Fujiwara-san,

    I am reviewing these screenshots now and will update you tomorrow. 

    Best regards,

    Layne J

  • 0 in reply to Layne J
    TI__Genius 12735 points

    Fujiwara-san,

    I have found this app note that contains some information you may find useful for DCAP parts. At the bottom of page 3 that document discusses how a load step up has a different impact on the regulator than a load step down. In summary, the load step up results in a relatively small undershoot because the voltage across the inductor is large in comparison to Vout (generally Vin-Vout > Vout) which allows the inductor to quickly make up for the current step. During the load step down the voltage across the inductor is only -Vout, meaning the inductor takes a longer time to adjust to the current step. This is specifically applicable to DCAP parts, although it is also generally true across all buck regulators.

    The output capacitors have a larger impact in the step down due to the higher ESR seen in the Aluminum caps. The higher the ESR of the output caps the longer the capacitors will take to absorb the extra energy stored in the inductor. You can also see the undershoot is slightly impacted by the higher ESR caps as well because it takes slightly more energy to recharge those caps after they are discharged during the load step up. 

    Let me know if you have any further questions.

    Best regards,

    Layne J