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Original question:

LMR38010: LMR38010 -HS AND LS SWITCH CURRENT LIMIT

LMR38010: How to decide the no of output capacitors required

ALPHONS PULICKAN
Prodigy 50 points
Part Number: LMR38010

Tool/software:

Hi All

  1. The output capacitor table for LMR38010 is shown above. For calculations involving a voltage conversion from 24V to 12 V at a frequency of 1000kHz , with an output current of 1A, a current ripple ration of 0.4(dI/Io), calculated inductance value is around 15uH, the minimum capacitance value for a load current change of 0.8 A and a voltage ripple of 3% approximately , calculated value of capacitance is around 1.11 uF, However the table indicates use of two 10uF capacitors. If the derated capacitance of a 10uF capacitor at 12 V and across all temperatures exceed the calculated capacitance of 1.11uF is a single capacitor sufficient ? How do we determine the no of capacitors needed in parallel ?

Formula used to calculate the minimum capacitance required is taken from TI app note SLVA477B

Thanks

Alphons 

  • 0
    TI__Mastermind 31085 points

    Hi Alphons,

    Mathematically, yes, it is sufficient, however we add additional output capacitance to minimize ESR, to improve the converter's immunity to transients, and to account for other cases such as manufacturing variances, etc..

    Usually a good starting point is 2x 10uF for this converter, since the load current isn't that high. We may sometimes add a lower capacitor value since they have a better response to higher frequency currents. 

    Thanks,

    Richard 

  • 0 in reply to Richard Fu22
    Prodigy 50 points

    How much ESR are we dealing with here?  Given that IC datasheet do not provide any frequency response characteristics, how do we determine the appropriate ESR value ? Value of capacitor is 10uF and we can get that in Ceramic capacitors and at around 1 MHz these will have self resonance and ESR will be very less (mohms), do we need to use parallel capacitors ? As designers how do we make this decision ? 

  • 0 in reply to ALPHONS PULICKAN
    TI__Mastermind 31085 points

    Hi Alphons,

    Sorry in delay in response. To determine the frequency response characteristics, you can check the capacitor datasheet. Usually ceramic capacitors have small ESR, so in order to get the benefits across the frequency spectrum, it is better to combine a big capacitor with something smaller, like a 10uF in parallel with a 0.1uF. 

    Thanks,

    Richard