Current source with pulse option
Since you still won't give us the obviously important information like what the digital output voltage is, what supply voltages are available, what the LED current is, and how much it drops at that current, I'm just going to make up stuff.

Here is a topology that should work:



The collector of Q1 looks like a controlled current sink when the digital signal is high.  The digital high voltage minus the B-E drop appears accross R1.  Since that is a fixed voltage, it causes a fixed current thru R1.  Due to the gain of the transistor, most of that can only come from the collector.  In this example, the voltage on R1 is about 2.6 V, so its current is 96 mA.  The LED current will be a little bit less, somewhere around 95 mA.

Note that the LED current is independent of the supply voltage as long as it is high enough.  Q1 has about 2.6 V on its emitter.  To act well as a current sink there should be at least 1 V C-E, so at least 3.6V on the collector.  In this example I assumed 1.9 V accross the LED when on, which gets us to at least 5.5 V supply.  A little more would be better so that the transistor is solidly into its current sink region.  The maximum supply voltage is only limited by the dissipation on Q1.  At 7 V supply, for example, it would dissipate about 250 mW when the LED is on.  If you are switching rapidly and using a average of 1/2 on time, then it would dissipate 125 mW at 7 V supply.

C1 is only there to speed things up a little since speed is a issue.  When the base is driven from the low to high state, C1 has to charge up.  This sends a little extra charge thru D1 at startup.  At turn off, it ensures the transistor is turned off as rapidly as possible.  The B-E junction will actually be reverse biased for a short time, which drains the minority carriers out of the base region quickly.  The value I show for C1 is a very rough guess.  There are too many unknowns to predict the requirements accurately.  The C1 value is best found by starting with a vaguely plausible value as shown, then find the right value by experimentation while watching the collector voltage on a scope.