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**Shunt Regulator Circuit Section: Selection of Peripheral Circuit Components**

2020.03.25 AC/DC

Up until the previous article, peripheral components of a synchronous rectification circuit section of the BM1R00147F power supply IC to be used were explained. In this article, we explain peripheral components of a shunt regulator circuit section.

Our discussion had focused on synchronous rectification circuits for some time, but please refer to this page to review the BM1R00147F constituent blocks.

**Peripheral Components of Shunt Regulator Circuit Section**

If the impedance is set too high, there is the possibility of excessive sensitivity to noise and other unstable behavior; in consideration of operation stability, R_{FB1} and R_{FB2} are selected such that I_{FB}=10 μA or so. Given the reference voltage V_{REF}=0.8 V (Typ) for the internal shunt regulator, VOUT is determined by the following equation.

In this design case study, VOUT is 5 V, and so R_{FB1} and R_{FB2} can be found using the following equation.

We see that because I_{FB}=10µA for VOUT=5 V, the sum of R_{FB1} and R_{FB2} is set to 500 kΩ, and the ratio of R_{FB1} and R_{FB2} should be determined such that the voltage at the branch point (SH_IN) is 0.8 V.

Next, C_{FB1} and C_{FB2} are determined. C_{FB1} is a capacitor for phase compensation; a value of about 1000 pF is selected. C_{FB2} is a capacitor for noise elimination at the SH_IN pin; typical values are in the range of about 100 to 470 pF. In this design case study, 220 pF was used.

The output load transient response can be adjusted using the resistor R_{SH1} to impart a bias to the photocoupler PC1. By using a small value for R_{SH1}, the output load transient response is made fast, and output voltage load fluctuations can be suppressed. However, there is a tradeoff between the load transient response speed and stability, and so thorough verification is necessary. In this design case study, a value of 510 Ω is used.

The resistance R_{SH2} sets the circuit current of the internal shunt regulator. When SH_IN=Low, the maximum value I_{SH_OUT_max} of the SH_OUT pin current is 75 μA. Hence the relationship between the Vf minimum value V_{f_min} of the photocoupler PC1 and the resistance R_{SH2} must satisfy the following inequality.

When V_{f_min} of the photocoupler PC1 is set to 1.1 V, R_{SH2} is calculated using the following formula to be 14.7 kΩ or less.

In this design case study, a value of 12 kΩ is chosen, including a margin.

This concludes selection of the peripheral components of the shunt regulator circuit section.

・The output voltage is set by setting the peripheral components of the BM1R00147F shunt regulator circuit section.