TL431 bias resistor

Hithesh

Genius 3290 points

Other Parts Discussed in Thread: TL431

How do you decide the bias resistor for TL431?

over 14 years ago

0 Ron Michallick over 14 years ago

TI__Guru**** 164796 points

Hello Hithesh,

Figure 1 in the TL431 data sheet (page 20) has a resistor. The Vka output goes to a load not shown.

The minimum value for this resistor is (Input - Vka) / (100mA + "minimum load current')
The maximum value for this resistor is (Input - VKa) / (1mA + "maximum load current')
Vka = 2.495V, Input = input voltage.

This will keep the TL431 cathode current with in the allowable range of 1mA to 100mA.

Regards,
Ron Michallick

0 Hithesh over 14 years ago in reply to Ron Michallick

Genius 3290 points

Ron,

I read the datasheet again after posting here.

I thought its pretty simple. My supply is 5V and Vref required is 2.5V.

Ik min is 1mA. I thought I'll make this 5mA.

So a 1K resistor should do it, right?

I don't understand what you mean by Vka output goes to a load not shown.

0 Ron Michallick over 14 years ago in reply to Hithesh

TI__Guru**** 164796 points

HIthesh,

The voltage on the 1k resistor is 2.5V (5V-2.5V); so the resistor current is 2.5mA. This is a good value.

The TL431 has 2.5V across it. Something in your application is connected to this 2.5V node (other than the 1k resistor). That 'something' is the 'load'.

Regards,
Ron Michallick

0 Saurabh Bansal over 13 years ago in reply to Ron Michallick

Prodigy 40 points

Hello Ron Michallick,

this calculation is for circuit of Fig. 1 in the TL431 datasheet, what if the circuit of Fig. 2 is to be used, how to calculate the R1 & R2 values.....

0 Ron Michallick over 13 years ago in reply to Saurabh Bansal

TI__Guru**** 164796 points

Saurabh,

Figure 2 has a formula for calculating the resistors. The constants (nominal) are Vref=2.495V and Iref=2uA.
For more accurate result use the equations in this app note. Setting the shunt voltage on an adjustable shunt regulator

Regards,
Ron Michallick

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TL431 bias resistor