Programmable Voltage Reference TL431IZT , what does it do? how does it work?

[danny davis]

Programmable Voltage Reference TL431IZT , what does it do? how does it work?

At my work they use Programmable Voltage Reference chips TL431IZT for the IC chips around a CPU microcontroller

They use a Programmable Voltage Reference TL431IZT to power a SC-531P Oscillator 8.0 MHZ

But why did they use a Programmable Voltage Reference TL431IZT , what does it do? why didn't they just use a voltage divider with resistors and trim pot?

If you put a lot of load on the output of a TL431IZT it doesn't drop the voltage? why is that? is it like a regulator?

What have you seen Programmable Voltage Reference used for? why did they use them?

 

[ysba]

It is like a zener diode, but you can adjust the "zener voltage" by choosing two resistor values. For the TL431, the minimum voltage is 2.5V. There's the TLV431, which voltage is 1.25 at minimum. You should use one when you need a voltage reference that doesn't change with temperature or supply voltage variations. If you use only resistors, if your supply vary a little, then your reference will vary too.

 

[danny davis]

But what is so different about it to use it? what does it do different than using a voltage divider or a zener diode set at a reference voltage?

 

[ysba]

They are very accurate. The voltage drop is pratically always constant. Simple zener diodes may vary a little with temperature rises, or from one single part to another. This regulators are made to be accurate. If the voltage drop should be 2.5V, it will be always 2.5V (of course, if you are operating it correctly).

 

[danny davis]

So they are regulators?

If the Voltage on the Input to the programmable voltage reference varies does it the output get affected?

If the LOAD gets more on the output on the programmable voltage reference does it vary or have a voltage drop?

How do you program this IC? not sure why it's called a programmable , how so it is programmable?

 

[ysba]

So they are regulators?

Yes, they are.

If the Voltage on the Input to the programmable voltage reference varies does it the output get affected?

A typical application is to connect the anode to the GND, and the cathode to the VCC via a pull up resistor. This resistor should be low enough to flow a current of a few miliamperes. While the VCC keeps a minimum amount enough to flow a certain small current trough this circuit, the output voltage will not vary, even if the input voltage is varying.

If the LOAD gets more on the output on the programmable voltage reference does it vary or have a voltage drop?

It will work just as a zener diode. But the purpose of this component is not to supply loads. Their power capabilities are very limited. They are usually just for generate a 'fixed reference voltage'.

How do you program this IC? not sure why it's called a programmable , how so it is programmable?

It's programmable by putting two external resistors. Their values will make a different output voltage, higher than 2.5V for the TL431.

Have you looked at the datasheets? There you can find a lot of typical circuits and how calculate the components ("program").

 

[danny davis]

I do have the datasheets, I'm just confused as To why someone would use these , for what use when you can use a voltage regulator or switching regulator instead.

When have you seen these programmable voltage references used for? for what kind of IC chips and for what kind of circuits?

 

[betwixt]

A conventional voltage regulator has a reference pin (usually ground or ADJ), an unregulated input pin and a regulated output pin. Their internal circuits hold a steady voltage between the output and reference pin by controlling the current they pass to their output pin. For many applications this is fine and they do a good job. Some applications do not need high currents or a series configuration is not suitable so a parallel regulator is often more applicable. The most common parallel regulator (aka a 'shunt' regulator) is a simple Zener diode but these are manufactured with fixed knee voltages. When a stable but variable shunt regulator is needed a TL431 or equivalent is normally used. It doesn't have an output pin like a conventional regulator, instead it draws a controlled current from an external source to try to maintain a constant voltage across itself. The voltage is set by the external resitor value. So in essence, it behaves like a programmable Zener diode.

Brian.

 

[ysba]

The TL431 is widely used at Switch Mode Power Supply (SMPS) circuits, when it is a isolated converter and the feedback comes from a optocoupler. You need to compare the output voltage to some known value so you know when to turn on the optocoupler.

PIC MCUs come with internal references, so you don't need an external IC like the TL431. But let's say you are using another MCU that doesn't come with internal references, and you want to have an AD reference different of VCC. You should use a TL431 connected to the AD reference pin, so your conversions will have a very accurate reference.

Another possible application is, for example, if you want to make a constant current source with a transistor and an input voltage that varies between 8 and 10V. In order to make a current source with a transistor, you need a fixed voltage at the base. If you use only a resistor divider, this voltage at the base will vary with this 2 V ripple at the input. You could use a zener diode at the base to fix the voltage you need, but let's say that there's no commercial zener diode for the voltage you want. You could try so a TL431, and have any voltage you want by adjusting two resistors.

 

[danny davis]

Thanks for the info.

So a TL431 is parallel regulator (aka a 'shunt' regulator) not a series Regulator?

It's output has a constant voltage by drawing current?

What is the difference between a series regulator and a parallel regulator?

 

[betwixt]

A series regulator goes in series with the unregulated supply, the output current passes through the regulator.
A Shunt regulator sits across the supply and draws current to pull the voltage down to the required level. It follows that a shunt regulator needs something in line with the power source to limit the current available to it. It controls the voltage across it by dropping the remainder across the limiter (usually a resistor).

A shunt regulator may at first seem to be wasting power but consider this scenario: A device needs a regulated 10V supply and your PSU is 10.5V, it will draw between 10mA and 20mA. A conventional series regulator probably wouldn't work because most need more than 0.5V difference between input and output to work properly. Instead you use a resistor to drop 0.5V at 20mA (25 Ohms). At 20mA that gives the correct voltage but at 10mA less voltage is dropped in the resistor and you device sees 10.25V which is too much. If you program a TL431 wired across your device to stabilize 10V, it will pass nothing (maybe a small leakage) when you draw 20mA and it will pass 10mA when you are drawing 10mA, in other words it keeps the current through the resistor constant at 20mA and hence the voltage dropped across it is constant.

Brian.