constant-currentcurrent-source
I need constant current 100mA source (120 channels of them) with as much low output tolerances as possible (so I don't want to use for example LM317 +resistors).
I found Supertex CL7 chip (+- 5%)
But there is problem, because I need to circuit my load one one side to GND. Here in this Chip load is between Vdd and Vout. Do you think I can circuit my load between Vout and GND?
Do you maybe have any other solution?
You mention that you want fast switching. None of the devices will be able to do that. Any the feedback circuits internally are designed to be slow so that they don't to much incorporate noise. And in fact some of the feedback is thermal feedback. Put another way, the dominant pole is low, 100's of KHz is typically the the fastest response time. This is true whether you are using a disable pin or connecting/opening the load to the source.
The best way to fix this is that you build an external circuit that current steers between two limbs. On one limb will be your LED chain and on the other a dummy load that draws the same amount of current (and hopefully also has similar load characteristics). An example of such a circuit is a differential pair. You should be able to get 10's of ns switching time then.
The challenge with the two limbs will be that the load characteristic differences may disturb the feedback of the LM317 etc. which then reacts slowly. If you can't match the two limbs then I'd suggest building a current mirror that decouples the current output the LM317 etc. so that it only sees a constant load the mirror transistor sees the variability.
You should be able to simulate all of this in LTSPice or similar before building.
The other factors you mention above not show stoppers, so pick what you want.
Yes it can be used as a constant current generator. With reference to the circuit: -
Take all the ground connections and group them together - this is now going to be the output voltage point for your load and the voltage across R2 (ADJ pin) will cause current regulation when this voltage is 1.24 volts. In other words if R2 is 10 ohms then 0.24 amps flowing into your load will be the current limit. R1 may be chosen to be quite low but the data sheet doesn't appear to say if this can be zero ohms. Maybe an experiment or more digging around will find an answer.
This method isn't limited to this regulator. Here's a 7805 regulator turned current source: -
Best Answer
Do you mean you want your Vout to be strictly 0v?
The chip you found have a constant current sink. If you have a current-driven device like LED, it's ok to let your Vout have some voltage like 0.2v or more. This does not affect your device. And for LED case the the current control the brightness and the internal resistance control the voltage drop across the LED. If u give 12v Vdd and voltage drop for LED needed is 3v the rest 9v must be eaten by the driver. This will make your IC very hot.
U can try TLC5940, it have 16 channel constant current sink and 120ma per channel.