Looking for design tips for low speed high current DAC

[monteie]

I am starting the design of a DAC for my application, but I have no idea of its feasible to implement in an IC given its large current output requirement.

The specifications are:

- Full scale output current of 500 mA
- Operates reliably at 20 mega samples per second
- Cannot use a linear amplifier at the output stage

The purpose of this device is to drive a string of high power LEDs that has a headroom requirement of 10 V. To allow for large headroom, I am looking specifically at open collector current output DACs. I would like to avoid the use of linear amplification stages in attempt to save power.

Does anyone have any insight on whether or not this is even feasible before I start pushing my transistors?

 

[kam1787]

no choice - use a mosfet or transistor driver stage

 

[dick_freebird]

Are you sure you want a current mode DAC? Or would you
be better off with a discontinuous conduction, PWM and
inductor "DAC"?

This latter is real uC friendly if you don't particularly need
feedback control of current but just a set-and-forget
kind of cal-map.

Any dimension can be digitally modulated; the design
values at play would indicate which one you'd prefer.

 

[monteie]

I originally considered the PWM DAC, however it seems getting one to function at a sampling rate of 20 Msps with a 500ma full scale current is not practical. Apparently I would need an HFET as the switch. Perhaps I missed something though. In the case of buck boost, I am assuming the switch must tolerate the 500 mA at a switching speed of 20 Msps, but perhaps there is another way to make it work.

 

[kam1787]

Why do you NEED 20 mega samples per second?!

 

[monteie]

With proper optical filtering, the high brightness LEDs can achieve bandwidths from 12 to 30 MHz (optical). Really, I only need 10 Mega samples per second, but the goal is to create a 100 Mbps link. I have achieved this using off the shelf components (such as 300 Msps video DACs and linear op amps) with a modulation depth of about 50ma. Given the dynamic range of these LEDs is around 700ma, I wanted to design my own DAC that can output the higher current with a lower bandwidth than a lot of high quality DACs.

Unfortunately, I have a constrained power budget and a linear output stage is not feasible. This is why I am attempting to design a high current DAC without a linear amplifier stage. Right now, I am assuming a current switched DAC might work (though these are known to also be power inefficient), or perhaps I can use a non linear high current driver on each branch of a different DAC architecture.