Jul 4, 2023
TI’s TL431 datasheet is amazing. Their engineers clearly love the over-40-years-old part. The examples section makes it sound like it can do nearly anything short of make you coffee.
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TI’s TL431 datasheet is amazing. Their engineers clearly love the over-40-years-old part. The examples section makes it sound like it can do nearly anything short of make you coffee.
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Post 23: TL431 Voltage equation Derivation
The datasheet of TL431 will fool you into believing that it is a simple IC. Well it is if you understand it correctly.
TL431 Voltage output equation explained
(Well I am not able to paste an image here. So just refer to the Figure 2 in TI’s TL431 datasheet!)
The output Volatge is Vka
The reference Voltage is Vref.
Lets Call the Current through R1 as Idiv.
Then Vka – Vref = Idiv * R1 ----- (eqn. I)
Idiv Splits into two Currents.
One as shown in the Diagram is Iref. Now Iref has been specified in the datasheet and has a current in the range of few micro-amperes. Not that it matters to our task in hand!
The Second Current is Ir2. Ir2 is the current flowing through R2.
So Obviously Idiv = Ir2 + Iref
But, Ir2 = Vref/R2
Thus Idiv = Vref/R2 + Iref
Substituting, equn derived for Idiv in eqn. 1
Vka = (Vref / R2 + Iref) * R1 + Vref
Vka = Vref * (R1/R2) + Iref * R1 + Vref
Vka = Vref( 1 + R1/R2) + Iref * R1
This equation exactly matches the one in the diagram.
Since Iref is very small, Iref * R1 is very small and thus its contribution is negligible.
The equation can thus safely be simplified to:
Vka = Vref(1 + R1/R2)
Important Points about the TL431 equation:
The programmed output voltage is not dependent on the input voltage. Of Course it is expected that the output voltage should not be dependent on the input voltage, since we are building a regulator.
The equation is dependent on the ratio of R1 to R2. And not on the absolute values of R1 and R2. This does not mean that you can select any value. More on this we will cover later.