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LM317A: LM317A Feedback Capacitance

Part Number: LM317A
Other Parts Discussed in Thread: UCC23513

Hi

We're using LM317A as the voltage regulation device in the motor driver application. (A noisy environment, there are so much electromagnetic Interference)

The LM317A is placed after the flyback transformer output to supply power to the upper arm of IGBT gate driver. (we use ucc23513, 50A IGBT) 

The picture as follows shows the LM317A circuit:

There are two ALCAP in the input(220uF) and output(100uF) of LM317A.

We wonder to know is it necessary to place a "uF" level capacitor in parallel to the feedback resistor(C150, R163) like datasheet? (TI suggest 10uF, we put 2.2uF here),

and Is it enough to reject the voltage ripple in this motor driver application?

Thanks. 

  • Hi, 

    The Capacitors on the ADJ pin are only present to improve PSRR. This typically helps in the <10kHz region.

    In looking at your circuit, please consider the tolerance of the ADJ pin current, Please refer to Eq. 1 :

    with R162 at 1K, and R163 at 12.7K, considering the worst-case leakage of the ADJ pin and the tolerance of VREF. The Voltage on the output could range from 16.44V to 19.08V.

    Reducing these by a factor of 10 would change the range to 16.44 to 17.94V. Also, this would help ensure that the minimum load current requirement is met:

    I hope this helps.

    Regards,