# TL431: Calculation of r & c for tl431feedback loop in dc dc converter

#### Arbitrage

Joined May 3, 2021
17
Hi I was designing a dc dc converter using LM5030

I was using tl431 and pc817 for designing feedback loop

I know two resistors r5 and r3 are used to set the voltage

but i have seen in many places c1 r2 and r4 are used, they are related to something type 2 compensator

I am a newbie to this and the materials available over the internet are confusing ,can anyone tell why there resistor capacitors are used and how to calculate them #### Arbitrage

Joined May 3, 2021
17

#### crutschow

Joined Mar 14, 2008
27,891
why there resistor capacitors are used and how to calculate them
They are used to help stabilize the feedback loop.
To determine their value, you need to calculate (or simulate) the feedback loop's response and determine the values need for proper phase and gain margin (typically with a Bode plot of the response).

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#### Papabravo

Joined Feb 24, 2006
16,938
Systems defined by 2nd order and higher differential equations are characterized by a rational polynomial called a transfer function. This rational polynomial will have some collection of poles and zeros. The location of those poles and zeros determine if the overall system will be stable or unstable. You need to be concerned with both the magnitude of the transfer function and the phase shift of the transfer function. For some combinations of magnitude and phase as a function of frequency a system may become unstable and oscillate effectively rendering it useless. Compensators modify the transfer function in such a way that this does not happen.

If you cannot determine or analyze the transfer function of the, system then there is not much hope of designing a suitable compensation scheme.

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