# Shunt regulator TLV431

Discussion in 'Electronic Design' started by j, Oct 7, 2008.

1. ### jGuest

Hello,

I'm using the TI TLV431 in the shunt configuration with two external
resistors (see schematic figure link below) to output +2V (vO), and an
Input of +5V. I've determined that the R1/R2 ration needs to be 0.613
(given that VREF=1.24V) for this to happen.

However, I'd like to determine sink resistance (output resistance?) of
this circuit but I'm not 100% sure that I'm doing it correctly.

My first take on this is that the output resistance would be
determined by shorting the output to ground and determining the short-
circuit current (isc), which would be completely driven by the +5V
input and the resistor Rin, thus the output resistance being: Rin. Is
this the correct approach?

Schematic figure: http://img115.imageshack.us/my.php?image=12965654es6.png
Datasheet for the TLV431 can be found here: http://focus.ti.com/lit/ds/symlink/tlv431.pdf

Any help would be appreciated.

Thanks!

2. ### PalindromeGuest

It really depends on why you want the "output resistance".

Normally, you will use this circuit to supply a range of output current
to other circuitry. I would measure the output voltage when sourcing the
min and max designed output current and derive the "output resistance"
from that.

3. ### jGuest

I need a low impedance voltage reference for the circuit linked below
(the red arrow in the image). I need a +2V since the output of the
AD629 has to be withing 2V of either rail. Do you know of a better
way to achieve this? I'm rather unfamiliar with voltage references/
voltage regulators. Note: +Vs = 5V, -Vs = GND in the attached
image. VREF would be the circuit referenced in my original post.
http://img509.imageshack.us/my.php?image=57289374jn8.png

The datasheet for the AD629 can be found here:

Thanks!
J

4. ### jGuest

awesome! thanks Jim!

5. ### PalindromeGuest

At first glance, it looks like a less than an ohm source impedance with
an over ten thousand ohm load impedance. So, I wouldn't be too worried

I think that you have misunderstood the significance of the "2v". This
chip, with a 10v supply, is incapable of producing an output voltage
lower than 2v or an output voltage higher than 8v. Without the
application of a 2v Vref, if used as a unity gain device, you would get
a constant 2v output for any input lower than 2v. Only after the input
exceeded 2v would the output start to change.

With 2v Vref, the output is still 2v when a zero input is applied, but
will start rising as soon as the input does - so, by the time the input
is 2v, the output will be 4v.

Obviously, once the output reaches 8v (with a 10v supply) it will stop
going up - no matter how much the input increases further.

So, it all looks fine. The only change I might make is to up the supply
voltage a tad. 5v is the minimum. I'd probably go for a 10 or 12v
supply, if one was available. I am always suspicious when manufacturers
quote one voltage range for operation, but use a rather narrower one for
important performance figures, like power supply rejection ratios.  