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LMQ61460-Q1: Input Capacitance

Inderjit Bains
Expert 2745 points
Part Number: LMQ61460-Q1

The datasheet (section 10.2.2.5) for LMQ61460-Q1 states that the minimum input capacitance is 10uF.  Questions:

1.  The users guide (SNVU698A) shows 2 x 4.7uF ceramic capacitors, UMK325B7475MMHT.  This capacitor does not have a capacitance vs. voltage spec, but similar capacitors show a 30-40% drop in capacitance at 24V.  Will 2x4.7uF work at 24V and higher? 

2.  Can I use a single 4.7uF ceramic capacitor in parallel with a 100uF electrolytic capacitor (EEH-ZA1H101P)?

3.  Currently the LMQ61460AASQRJRRQ1 is only available from TI.  When will it be available at distributors?

  • 0
    TI__Guru 50120 points

    Hi Inderijit,

    1. Since the user's guide shows the EVM with an input ceramic BOM of 2 x 4.7uF, this will for up to 36Vin. If you expect some type of load transient then I'd suggest increasing the input capacitor value to reduce the voltage ripple and further maintain input voltage during transients. 

    2. If you are using only one ceramic, I'd suggest bumping it up to 10uF ceramic and then having it paralleled with the 100uF electrolytic. Also I'd suggest adding two high frequency bypass capacitors ; one on each since of the input/ground pin pair (VIN1/PGND1 and VIN2/PGND2). 

    3. Let me circle back with the marketing team to see if there is any estimate timeline for availability.

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Expert 2745 points

    1.  The capacitance deceases with applied voltage for ceramic capacitors.  At 24V, it's typically 40% less than the rated capacitance.  In this example, 4.7uF x 2 x 0.6 = 5.6uF.  With 10% tolerance it may be down to 5.1uF.  This is not the 10uF value asked for in the datasheet.

    2.  The question should have been:  is the requirement 10uF of ceramic capacitance (very low ESR)?  As mentioned above, a 10uF ceramic may be down to 5.1uF at 24V.

  • 0 in reply to Inderjit Bains
    TI__Guru 50120 points

    Hi Inderjit,

    1. Yes I agreed that there will be some DC bias affects on the ceramic capacitor. However as stated previously since the EVM is using these values and the datasheet has efficiency curves using this EVM, the 2 x 4.7uF will suffice. The 10uF in the datasheet is more of a typical value rather than after derating.

    2. There is no hard requirement for 10uF. The more input capacitance the better the input voltage will maintain during load transients. If there is a choice given the same package size between a 4.7uF and 10uF, I'd suggest the 10uF. Otherwise 4.7uF ceramic with a bulk electrolytic would also suffice.

    Regards,

    Jimmy