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UC1708-SP: Bypassing for the UC1708

Kenneth Aquino
Expert 1300 points
Part Number: UC1708-SP
Other Parts Discussed in Thread: UC1708

The UC1708 datasheet shows a 10 uF capacitor

in parallel with a 47 uF tantalum capacitor connected from the chip's VCC to ground.

How do I determine what the IC needs in my application.  I only one of the two drivers in the the UC1708 and am driving a MOSFET with approximately 2000 pF of Ciss at 450 kHz.

Ken

  • 0
    TI__Mastermind 24875 points

    Hey Ken,

    I'll help give you my best estimate and try to explain my process.Hopefully you can figure out what you need after you see my approach.

    Assume you are driving up to 20 Vs and have a Ciss you explained.

    This would mean the charge that you need is = 2000 pF * 20 = 40 nC

    Assume that you are using the 57 uF of cap at the input of the device, the change in voltage because of the capacitance is:

    40 nC/57 uF ~ 0.7 mV

    With around 10 mOhms ESR change from resistance would be:

    0.01 * 3 A (max current spike) = 30 mV.

    ~30.7 mV of change doesn't sound that terrible and fairly stable for a 20 V supply. 

    Notes and Warnings:

    A slower drive will change the spikes from ESR quite a bit.

    I didn't factor in ESL which will likely also come in to play, but most caps don't spec this. This means that the ceramic 10 uF is going to be very important and probably cause some amount of error from calculated.

    All in all I believe that a 10 uF cap plus a 47 uF tantalum would likely give the needed bypassing for the example.

    Get the ceramic capacitor as close to the Vin supply and return ground as possible. This is critical for reducing noise and ground loops that will cause you trouble.

    Thanks,

    Daniel