I'll maybe use some TL431s as regulators for my future low power projects (1st on the list: dac 
), and I have some basic questions about theyr use.
With the few electronic skills I have, I understood that the shunt reg regulates the voltage by eating more or less current, on the voltage sources that feeds the design to regulate.
To regulate the voltage by eating current, the source has to have a high impedance, so we add a resistor in series to the voltage source.
Now, how to calculate this resistor's value?
If we take the example of a TL431 (max=100mA), regulating the voltage of a TDA1543 (typ=50mA, max=60mA, min=??)
I was thinking that:
-if the TDA1543 is at it's maximum (60mA), the TL431 can shut down if the resistor's value is calculated to have 5V on the high impedance voltage source when 60mA are used.
Let's say we have a 6V source, that gives a 16.67R resistor?
-now, with this resistor, if the dac draws no current, the TL431 will have to draw the required current to have a 1V voltage drop on the resistor (to obtain 5V on the chip)=60mA (lower that 100mA, the limit of the TL431, that's OK)
In other words, the drawn current has to be constant, I(TL431)+I(load)=constant. A kind of class A power supply
But since the TL431 only supports 100mA, the load must have a I(max)-I(min)<100mA
Am I right?
), and I have some basic questions about theyr use.With the few electronic skills I have, I understood that the shunt reg regulates the voltage by eating more or less current, on the voltage sources that feeds the design to regulate.
To regulate the voltage by eating current, the source has to have a high impedance, so we add a resistor in series to the voltage source.
Now, how to calculate this resistor's value?
If we take the example of a TL431 (max=100mA), regulating the voltage of a TDA1543 (typ=50mA, max=60mA, min=??)
I was thinking that:
-if the TDA1543 is at it's maximum (60mA), the TL431 can shut down if the resistor's value is calculated to have 5V on the high impedance voltage source when 60mA are used.
Let's say we have a 6V source, that gives a 16.67R resistor?
-now, with this resistor, if the dac draws no current, the TL431 will have to draw the required current to have a 1V voltage drop on the resistor (to obtain 5V on the chip)=60mA (lower that 100mA, the limit of the TL431, that's OK)
In other words, the drawn current has to be constant, I(TL431)+I(load)=constant. A kind of class A power supply
But since the TL431 only supports 100mA, the load must have a I(max)-I(min)<100mA
Am I right?
You're pretty much right. Under no load conditions you have correctly calculated value for Rb which will aloow 60mA to flow through the regulator. Under full load though, the 60mA may flow through the load and not enough current flows through the regulator. A reasonable current for the 431 is around 10mA (i think) so you should recalculate Rb assuming total current is 70mA.
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