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TL431 R1, R2 resistor value

Other Parts Discussed in Thread: TL431

Hi,

I have a question about TL431A.

[Setting Information]

Cathode Voltage: 5V

Cathode Current: 5.75mA

R1: 100k Ohm

R2: 100k Ohm

Load Capacitance: 10uF (capacitance in stable operation)

 

As the result of operaion test, output oscillation was appeared.

 

When I change resistor value for R1 and R2 to each 10k Ohm,  the output oscillation was disappeared.

When I add an output capacitance to 5uF (total 15uF), the output oscillation was disappeared.

 

Could you advise me why it appears output oscillation when I use 100k Ohm resistors for R1 and R2?

 

Best Regards,

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  • Hidetoshi-san

    The TL431 and circuit board will cause a small capacitance from Reference to ground. (Feedback pole)
    A small capacitor from cathode to reference will help. Use a value of C=1/(3.14*F*R). F is oscillation frequency. R is 100k (R1). C provides a zero at 1/2 the oscillation frequency for a phase margin boost. Round 'C' up to the next standard part value. 

    Using 15uF decreases the bandwidth of the TL431 and reduces the phase lag in the feedback pole.