TL431: PNP transistor in high current shunt regulator application

Hi Nikita,

In this application the TL431 helps with regulating Vout with T1.

As you noted there is 3 key factors when designing this type of circuit.

First is Vout.

Vout: Vref * (1 + R1/R2) + (Iref * R1)

The feedback loop that regulates the output now includes T1.

Second: R4 should be sized on the maximum current you need at the max load + ika(min) + margin

R4 = [Vin(min) - Vout] / [ILoad(max) + Ika(min) + margin]

Vin(min) = the minimum Vin where you want the output to regulate.

Third: R3

Because during a no load condition, all the current will be divided between Ika + Ic.

R3 should be sized based on the maximum Ika you want to pass through the TL431.

VBE( at Vin(max) & ILoad(min)) / ( IKA(max) + Ib) < R3 < VBE(at Vin(min) & ILoad(max)) / Ika(min)

The current will always be the current that is not through the load + Ika. So choose a T1 that can handle the worst case current

Ic = (Vin - Vout) / R4 - Ika - ILoad.

Ic(max) = (Vin(max) - Vout) / R4 - Ika