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LMR33630: Load transient behavior

Hampton Hsia1
Prodigy 20 points
Part Number: LMR33630

Hi, 

I found that the duty cycle would not change when the load transient happened.

That caused the DCDC took more time to settle and the output voltage rose higher than I expected. 

The datasheet told me that the duty cycle would automatically change in different conditions.

So was this behavior correct?

CH1   Yellow   Vout(5V)

CH2   Bleu      SW

CH4   Green    Load current(20uA~2.5A)

Thanks,

Hampton

  • 0
    TI__Expert 8245 points

    Hi,

    If you mean the load changes from light load to heavy load, from you waveform the duty cycle is obviously changed because the period becomes much longer.

  • 0 in reply to Iven Su
    Prodigy 20 points

    Hi, 

    Please check the place circled by the red line.

    I mean the DCDC left the PFM mode and entered into PWM mode when load changed from light load to high load, the duty cycle did not change.  

    Thanks,

    Hampton

  • 0 in reply to Hampton Hsia1
    TI__Expert 8245 points

    Hi,

    If you mean the device is already on PWM mode, then if you change the load from a heavy one to another heavier, the duty cycle will have a very small change due to the voltage drops on switch, DCR of inductor and traces changing by the load. Because the voltage drops is very small comparing output voltage, then duty cycle also changes a little, which is hard to be observed on the waveform.