DAC controlled current source / lt1206

[stcoso]

Hi

I'm trying to desing a little amplifier to control some LEDs with square wave from a DAC. LEDs will draw 700mA max (Vf around 20V).

With my really limited experience  i thought that something like the attached schematic would work but i'm keen to expand my horizon  .

So I begun looking for an op amp that would work in my schematic... and I found some ICs (lt1210, lt1206) that seems to be capable of directly driving my load. Where i'm wrong  ?

[exe]

I just opened lt1210 datasheet, it's a current-feedback amplifier. I'd say it's nothing to do with current sourcing. It's sort of a "normal" opamp except the negative feedback works quite different than conventional (voltage feedback) amplifiers. Datasheets tell a bit about their applications (like driving capacitive loads and long cables).

I think youe schematic is a bit wrong. The load should be on high side, and LOAD1 should be a current shunt. Also, I'd put some compensation. Be sure heat dissipation is under control: 4V*0.7A = ~3W (dissipated on shunt and pass transistor), heatsink is needed. Also quite close to maximum ratings of bd139 (but still fine).

[stcoso]

I put the load on the low side because i thought using the BJT as a common collector amp.



EDIT: I know that those two are CF op amp (and i'm gonna modify the circuit accordingly)... I'm just asking if they can be appropriate for my purpose 

[exe]

I wouldn't use them because they are expensive, too fast (=can oscillate if you are not careful with layout, bypassing, etc), and can be harder to cooldown. LT1206 only gauranties to produce 0.25A, not sure under what conditions it can output more. I'm also not sure what the schematic would be (where to place shunt?), but it's definetely not the same.

My advice: don't go with these devices. I'd just use a typical "current sink" schematic, with compensation. BTW, if base current flows through the shunt, this gives some error as this current is doesn't flow thrught the load (this is only valid for bjt pass transistor, fets are fine, but not recommended for linear application due to Spirito effect, although people use them).

[David Hess]

A power operational amplifier could drive your LED load directly but the example you show is generally a better way.  Slightly more clever is to directly replace the output transistor with an adjustable voltage regulator like an LM317 to add fault protection.

[gf]

I put the load on the low side because i thought using the BJT as a common collector amp.

A common collector amplifier has low output impedance, and the negative feedback via the opamp makes it even lower. So it is a voltage source, not a current source. In order to drive the LEDs with constant current, put them between collector and V+, and put a current sensing resistor from the emitter to GND. Furthermore I'd renounce the R1/R2 divider and connect the emitter directly to the opamp's IN-, in order to reduce the required voltage drop at the current sensing resistor. The current through the LEDs will be approx. DAc's V_out / R_emitter then.