Looking for design tips for low speed high current DAC

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?

no choice - use a mosfet or transistor driver stage

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.

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.

Why do you NEED 20 mega samples per second?!

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.