>... driving a string of LEDs where the high end
> is at +32VDC. I' looking at driving the low end through a resistor to
> limit current.

Better op amp and transistor adjustable sink to set constant current.
O/C to turn off (se below).
When dumping load on his you have to be sure that response time is Ok
to stop Iovershoot_iniial going too high. Simple matter of engineering
:-).

> I COULD do open collector or open drain, but I'm
> thinking that the LEDs could go off faster if I actually drove them off.
> So, I'm thinking of driving the bottom of the current limit resistor with
> 0V to turn the LEDs on and +5V to +10V to turn them off. Switching time i=
s
> important because I'm using up to a 3MHz square wave.

>=A0I COULD do open collector or open drain, but I'm> thinking that the LED=
s could go off faster if I actually drove them off.

Active offg as described should do no harm, but ...

LEDs have  fast response - unless you have external capacitance across
the LEDs  you are unlikely to get much useful gain by active level
drive compared to open collector on/off.

Quick let's-see.

Say Von =3D 30V for now
Assume 100 mA operation (you said 200 mA driver).
LED power =3D 3 Watt.
To operate for 1/3 uS =3D 3 x 1/3 x 10^-6 Joule =3D 1 uJ

Capacitor at 30V to store 1uJ =3D 0.5 C V^2
C =3D 10^-6 J x 2 / 900 ~=3D 2.5 nF.

This has the energy to operate the LEDs for 1/3 uS but would not do so
for anywhere as long as V droop will rapidly drop you below
V_LED_operate. Actual on time will be quite a lot less.

So unless the capacitance ON BOARD on the LED board across the LEDs is
>> 2.5 nF they will be off in << on 3 MHz cycle. Shorter at less
current.

E&O as ever.

Phosphor glow is << 1/3 uS.



    Russell McMahon
    Applied Technology ltd
    New Zealand

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