It's NOT a zener, or a diode, or even a voltage source.
A much better way to think of these devices is:
An op-amp, self powered, with open-collector output, and a large, precise input offset voltage.
The REF pin is actually -in, and it's offset by 1.24V or so (2.5V for the regular TL431).
VCC and output are the "cathode" pin. Unlike an op-amp which can source or sink current, this can only sink current.
VEE and +in are the "anode" (common) pin.
Another way to look at it is, a transistor with precision Vbe, very high beta (>> 1000), and low fT* (~1MHz).
*Not precisely "fT", because of the high beta. Voltage gain, or gain-bandwidth product (GBW), would be more accurate.
Now, as an op-amp, you do have to worry about feedback effects. Fortunately, it is unity-gain stable, so you don't have to worry too much in typical applications. Example: REF strapped to K gives a 1.24V reference. Because of the limited gain (especially at high frequencies), this reference will have a constant voltage characteristic at low frequencies, becoming inductive at higher frequencies (where the gain isn't high enough to stabilize the voltage against changes). As an inductance, it can resonate with a capacitance, so you mustn't connect an unstable amount of capacitance across the device. Which is familiar from regular op-amps: they don't usually appreciate capacitive loads.
If you want a higher voltage, no problem, just connect a voltage divider from K to REF to A. This is precisely the same as a non-inverting amplifier circuit. The output impedance and noise level both rise proportionally, of course.
If you use the device in a feedback loop, then you probably need to connect compensation from K to REF. Usually, an R+C is connected here. Then REF is connected to a voltage divider which senses the output, and K connects to the rest of the control circuit. TL(V)431s are frequently used in switching supplies for this reason. It's an error amplifier AND reference, all in one!
Tim
Home
Help
Search
Login
Register
