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LMZ14203: 100mV ripple, and strange transient response

Hendrik Lipka1
Prodigy 20 points
Part Number: LMZ14203

I have a LMZ14203 with a circuit straight out of the data sheet. The two differences are that my Rfb resistors are of higher value (12.3k/2.2k for 5.15V output), and the Ren divider is of lower value (10k / 1.5k for 8V). Local input capacitance is 10µF X5R, output is 22µF X5R.

I'm observing about 120mV ripple on the output with no load and 100mV with 30mA load. What is strange though: when I step up from no load to 30mA, the output voltage rises from about 5.2 to 5.4 (with the added ripple). Is this to be expected?

Everything looks OK with my target load of 2A, but during startup I expect low load and 5.5V is somewhat on the high side.

  • 0
    TI__Mastermind 18940 points

    Hi Hendrik, 

    The higher ripple and bump in regulation is expected at light load. When the load is low and the part is in DCM mode of operation, the switching frequency will decrease to minimize the switching losses and maximize conversion efficiency. The tradeoff is higher output ripple voltage (because of the lower frequency) and a bump in the regulation. 

    You may be able to optimize this by changing the Ron resistor value and running at a bit higher frequency.

    If you need to improve this further, the LMZ14203H (usually used for higher output voltages, pin to pin compatible with LMZ14203) may be a better option. It still provides you with the switching loss savings at light load but also improves the regulation at light load. 

    Cheers, 

    Denislav

  • 0 in reply to Denislav Petkov
    Prodigy 20 points

    Thanks for providing this insight. Good to know that this is normal behaviour - I would like to see this mentioned somewhere in the data sheet. With a 56k Ron (instead of 100k as in the data sheet) the ripple is a tad lower, and the voltage increase also a little bit lower. I additionally changed the feedback divider, so that I won't run with more than 5.4V output

    Many thanks,

    hli