i believe the maxim.com site has a design for just such a circuit.

David VanHorn wrote:
>
> Is anyone aware of a linear regulator, similar to the LM317 or 338, that is controllable by several bits of binary input?
>
> I know I can do it in external hardware, with a 317 or 338, but I'm looking for a minimum component implementation.
>
> Switchers are viable alternatives, but in this case, efficiency is not a plus.
> I'm actually looking to use them as controllable current sinks, and dissipating watts is what I want them to do. :)
>
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