> I've wondered about white LED's which I think use a phosphor (could be wr=
ong
> there) ~ does anyone know if that adds any measureable/useful amount of
> persistence? So far I haven't seen any spec on this in datasheets.

The large majority of White LEDs use blue LED and yellow phosphor.
BUT the phosphor response time is extremely fast.
<<  1 uS range which is well above what is useful here.

 R

_________________

Relevant comment here.
Bottom of page.
      http://cr4.globalspec.com/thread/64849/White-LED-with-Shortest-Respon=
se-Time



Just to put a number on it, it's easy to push LED response down into
the 40ns territory, but I experienced frustration trying to get them
to turn off faster than 20ns. This was true even when strongly
reversing the current within under 5ns.

Below 20ns it becomes a matter of finding the right LED. I've seen
claims of results down to a few ns, but they didn't report what part
they were using. I tried many parts and wavelengths, etc., without
success. It's like the proverbial fish that got away.

But laser diodes, OTOH, are very fast. They're happy to respond in
under 1ns, so it becomes a matter of the driving electronics and fast
low-impedance wiring, etc. But it might be painful trying to create an
apparent white light with multiple laser diodes of different
wavelengths. They also naturally make beams rather than flooded light,
greatly restricting their applicability for ordinary lighting.

White LEDs are often driven with dc-dc converters to control the
current. The voltage step-up types, which can drive several 3V LEDs in
series, are especially convenient. But most of these are fairly slow.
One exception is TI's TPS61166 (link), which can turn the LED on or
off in well under 1us.

TI's motivation for such a fast response time is to insure a linear
light-level vs PWM duty cycle, e.g., down to the 1% region, even with
high PWM frequencies like 40kHz. For example, 1% of a 40kHz 25us
period is 0.25us, so they would benefit from a response time faster
than that. Although their boost-converter runs at 1.2MHz, this is not
fast enough for the desired switching rate, so the TPS61166 adds an
additional series FET switch to drive the stack of LEDs.

You can add an external MOSFET to an ordinary boost converter, to
obtain the same fast result as TI's part. It doesn't appear that
distributors are stocking the TPS61166, so that may be the best
approach.
--=20
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