Why the DAC is not used to drive the optocoupler .
My application is to dimming of AC load using Optocoupler.

 

study the opto performance versus dc input.
After this study you know why.

 

Why the DAC is not used to drive the optocoupler .

Why do you think DACs are not used to drive optocouplers?

 

I saw that many of peoples uses pwm to control the optocoupler driving

 

You have phase shift a AC load pwm will not give a even dimming of power.

 

You have phase shift a AC load pwm will not give a even dimming of power.

Why not?

 

See Fairchild app note AN-3006.
Max.

 

Why the DAC is not used to drive the optocoupler .
My application is to dimming of AC load using Optocoupler.

The IRLED in the opto isn't linear - a couple of the big name manufacturers have published appnotes for linear opto drive.

Basically; your signal source would drive two opto IRLEDs in series - the transistor in one of them is included in a nfb loop on the IRLED drive circuit, while the other opto transistor drives your target load.

I'm guessing the AC load means you're using an opto triac, that's probably a bit more tricky but it should be possible to work out some sort of compromise.

 

For AC load the Fairchild app shows both phase angle switching and zero cross switching via opto triac driver.
Max.

 

If read the appnotes to pwm the Phototriac you have to get timing right which is what I found 6 years ago when I tried pwm to dim lights that was on 120 AC if you got the pwm right you could dim the lights some what but it was never as good as Zero–Crossing

The critical rate of rise of off-state voltage, dv/dt, is measured with the input at 0 V. The frequency of Vin is increased until the phototriac turns on. This frequency is then used to calculate the dv/dt according to the formula: The critical rate of rise of commutating voltage, dv/dt(c), is measured by applying occasional 5-V pulses to the input and increasing the frequency of Vin until the phototriac stays on (latches) after the input pulse has ceased. With no further input pulses, the frequency of Vin is then gradually decreased until the phototriac turns off. The frequency at which turn-off occurs may then be used to calculate the dv/dt(c) according to the formula shown above