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Original question:

UCC33420: UCC33420 noise filtering

UCC33420: Maximal capacitance load.

Vassili Ivanov40
Prodigy 220 points
Part Number: UCC33420


Tool/software:

I need a very low noise -5V 160mA max power rail

I use UCC33420 (in 5.5V output mode) followed by TPS7A3301 LDO.

What is the allowed maximal capacitance load?

I have 22uF * 5 before LDO.

  • +1
    TI__Genius 15616 points

    Vassili,

    You can use the "Soft Start time and steady state burst on duration calculator" tool to calculate the Cout max that UCC33420 can handle before triggering t_on max fault. Link of the tool is below. 

    UCC3342X-CALC Calculation tool | TI.com

    I went ahead and calculate it for you. Assuming, VIN=5V and VCC=5.5V, Iload=260mA with a VCC_threshold=2V (UVLO of TPS7A3301), the Cout max that the device can handle if 72uF. Something to be aware of here is that even though you are using 5x22uF=110uF total Cout, you need to check the Cout @ 5V Bias, which is different than Cout @ 0V Bias (110uF). The calculator differentiates both values and the value that matters is Cout @5V Bias. I recommend checking this value in the DS of your caps and if it is higher than 72uF, you will need to reduce Cout so that Cout @5V Bias is <72uF.

    Attached is the calculator Design that I did for you.

    UCC3342X-CALC-V1_PhotoSound_Vassili.xlsx

    Thank you