bandgap current reference
wjaachen said:
The current of Bandgap is heavily dependent of temperature.
That is not true. It is the case in the typical bandgap structure, but if you understand how a bandgap works, you can make anything you want. Remember:
A bandgap is a linear combination of vbe voltages You can add these voltages any way you want it such that you get the temperature dependency that you want either for V and I.
OK. The most typical configuration is to subtract two vbe voltages with a certain ratio. That gives you a voltage that is PTAT,
Proportional
To
Absolute
Temperature. Then you add to that another vbe voltage (negative temp coefficient) and you end up with a
voltage that is independent of T. You can see that in your circuit there are voltages dependent on T, on 1/T and independent. Of course, if you put the flat voltage over negative T resistors, the current that you get acts the same way vs T.
Now, what do you want?
* A current that is independent of T and you don't care about the voltage
* A current that is independent of T and a voltage that is independent too
If it is the first case, what you have to do is to make the linear combination such that the current is flat with T and not the voltage. You make that changing the ratios of the resistors (Rptat, Rvbe, Rref...)
If you want both to be independent of T, you first make usual bandgap. Then you have a voltage that is flat with T and a current that depends on temp. Now look for a voltage in your circuit that has opposite t coeff than your current. Use a small amplifier to force that voltage over a resistor. Add the two currents. What you have to do is to change the resistor value until the current is also flat