CCS values

[donovas]

Hi, sorry for such trivial question but im trying to sink about 1.5ma current from parallel tda1387 arrangement with this circuit. The npn will be bc550. What should my voltage input and resistor values be?

Thanks in advance.

An externally hosted image should be here but it was not working when we last tested it.

 

[Mark Johnson]

The TL431 datasheet cautions you to (a) never apply more than 36 volts across it; (b) run it at a cathode bias current greater than 1mA and less than 100mA. In your circuit this means you want VI(BATT) less than 36 volts, and you want

• R > VI(BATT) / 0.1A
• R < [ VI(BATT) - 2.5 ] / 0.001A

If you can't connect the resistor straight to your positive power supply, because your power supply voltage is more than 36 volts, then what? You can always build a resistive voltage divider to create a node voltage that's below 36V, and use that node voltage to bias your TL431. Another way to skin the cat is to build a 20V shunt regulator using a resistor and a zener, then bias your TL431 from the 20V point.

 

[Ketje]

No more than 50V with a BC550.As long as the transistor is ok the TL431 sees no more then the Vref (2V5) + Vbe (~0V6), not to worry.
The resistors are as the picture shows, giving the TL431 a 1m5A bias to be on the safe side of the minimum 1mA of the datasheet.
Mona

 

[donovas]

thank you so much

 

[nigelwright7557]

I would have used a 2 transistor CCS.
Using MPSA42 high voltage transistors.

 

[Ketje]

I would have used a 2 transistor CCS.
Using MPSA42 high voltage transistors.

+1
Some people like it complicated, like a 0.1% resistor at a reference of ±0.4%
Mona

 

[nigelwright7557]

+1
Some people like it complicated, like a 0.1% resistor at a reference of ±0.4%
Mona

I just thought a TL431 was over kill.
All my designs get away with 2 transistor CCS.