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LM27762: Question about the capacitors in the LM27762 EVM

Lorenzo Marcantonio
Prodigy 190 points
Part Number: LM27762
I'm studying the evaluation board to reproduce it in out product and I noticed an interesting thing: the input bypass (C2) is specced at 1µF/25V, while the flying capacitor (C1) is 1µF/10V. The input tank/bigger bypass CBAT is 10V, too. Why is C2 rated 25V? is derated for the capacitance loss due to the biasing or to handle the overvoltage made by the wiring inductance after the initial charge rush? Since CBAT is rated only for 10V I'm leaded to the first hypotesis but since CBAT is ceramic anyway shouldn't be the capacity loss acceptable? Thanks in advance
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    TI__Intellectual 880 points
    Hello Lorenzo,
    the 1uF input capacitor (C2) 25V versus 10V rating should provide lower ripple and noise at your input supply rail. Depending on your application requirements you can also test a 10V type. However it is important that also the bigger 10uF capacitor (CBAT) is located very close to the VIN pin of the IC as at least 2uF effective capacitance is required.
    Please read section 8.2.2.5.2 in the data sheet which provides you more hints how to select the input capacitor.
    Best regards,
    Ivo