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LMZ20502: Output voltage waveform discussion

Part Number: LMZ20502
Other Parts Discussed in Thread: TPSM82822

Hi team,

It is nice to know you here. My customer used LMZ20502 as below, but the output voltage has a very strange rise edge at start-up. Could you please share with me what may cause this? Thanks.

BR,

Charles Lin

  • Hi Charles,

    Thanks for sharing the schematic and waveform.

    Is there a load, the real system load or an electronic load, connected during this time?  If so, how does the start-up look when that load is not connected?

    Is there additional output capacitance on this 2.5V rail, which is shown on other pages of the schematic?  How much total capacitance is present?

  • Hi Chris,

    Thank you for your response here. The load is smaller than 100mA, the total output capacitance should be larger than 150uF. Could you explain what may cause this? Thanks.

    BR,

    Charles Lin

  • Thank you for confirming.  Is an electronic load used?  Has the 100 mA been confirmed through a measurement?

    You are likely triggering current limit during start-up.  The current limit is reduced during start-up, as explained on page 10 of the D/S.  The D/S also gives this recommendation for large output caps, such as 150 uF:

    The maximum value of total output capacitance should be limited to between 100 µF and 200 µF. Large values of output capacitance can prevent the regulator from starting-up correctly and adversely affect the loop stability. If values in the range given above, or larger, are to be used, then a careful study of start-up at full load and loop stability must be performed.

    Why do you use so much capacitance on the output?  Can you remove some of it and see if the start-up improves?

    As well, we have the newer, smaller, cheaper TPSM82822 which does not use a current limit based startup.

  • Hi Chris,

    We don't use the current probe to do the measurement. If the load current is very small, will the output voltage behave like this?

    The output capacitor value is about 150uF, which is in the range you recommend here. The customer also uses this device to generate 3.3V with an even higher output capacitor, that device does not have such an issue. So we do not think this is caused by a large output capacitor. 

    Do you have some other explanation here? Thanks.

    BR,

    Charles Lin

  • Hi Charles,

    You will need to perform some measurements to answer your question.  Please measure the output current at start-up.  You could also try removing some capacitance and seeing if the waveform improves.