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LM4938 input/output cap value

Shu-Cheng LIN
Guru 11785 points
Other Parts Discussed in Thread: LM4938

Hi Sir,

Sorry to bother you.

We have use LM4938. 

At datasheet page 3  fig3, Pin10 and Pin12 CIL are 0.33u, Pin17 and Pin26 are 220u

We need to know how to calculate this value. Could you help explain and if have doc for us is better.

Thanks for your help

  • 0
    TI__Guru** 109825 points
    Hi Shu-Cheng,

    The input capacitors, on Pin 10 and Pin 12 produce a high pass filter with the input resistors, the cutoff frequency can be calculated with the following formula:
    fc = 1/(2*pi*Ci*Ri)
    For the example on figure 3, Ci = 0.33uF, Ri = 20kOhm so fc = 24.114Hz. There is some information about this on page 18 in "INPUT CAPACITOR VALUE SELECTION" section.
    The output capacitors, on Pin 17 and Pin 26 produce a high pass filter with the load resistance, the cutoff frequency is calculated with the same formula as the input components:
    fc = 1/(2*pi*Cout*Rl)
    Where Rl is the load resistance, it could be a line output which is around 600Ohm and 1.5kOhm or it could be a speaker around 4Ohm. That is why this capacitor is bigger than the input capacitors. For the situation of having 4Ohm load, the cutoff frequency would be:
    fc = 1/1/(2*pi*220*10e-6*4) = 180.85Hz

    I hope this helps. Let me know if you have any more questions.

    Best regards,
    -Ivan Salazar
    Texas Instruments