Hi there ian,
If you're looking for max efficiency you'll
need to use an actual dc-dc converter circuit
unless the voltage input is close to the voltage
output in which case you may get away with an
LDO regulator.
Pulse width modulation, by itself, does not
help efficiency at all. It has to be part of
a circuit that contains at least one energy storage
element and the circuit has to be arranged correctly.
Many people have argued time and time again that
PWM through a smaller value resistor saves
energy somehow, but they never include the math
to back it up because the math shows that there
is no gain in efficiency, and in fact the eff
is the same as when using a single series
resistor to set the LED current.
There is the possibility however that you only
want to PWM to keep the brighness of the LED
at a constant level, which is fine, but dont
expect eff gains because you're pulsing.
One additional point:
Current regulation based on the HFE of a
transistor is a bad idea due to the variations
in HFE over temperature and from device to
device. I guess if you're only building one
and the ambient wont vary that much maybe you'd
be ok, but i'd use a resistor in series with
the LED, unless the input voltage can fall too
far, and then i'd use a simple current regulator.
If you really do need the eff, then you're stuck
using some sort of dc-dc converter that actually
*converts* power (has probably an inductor).
From the sound of it, you'll need a buck
converter because you're LED voltage is less than
the power source.
If you are driving a 3.5v LED from 6 volts, you'll
note that the battery isnt always 6 volts but
rather has some average value like 5 volts so
if you use a regular current regulator you'll
lose some power but not as much as if the power
source was always 6 volts.
If you're drving from 9v then you want a buck
circuit.
In any case, using PWM wont save you anything
at all over a linear regulator because as mentioned
before PWM by itself does not increase efficiency.
This means you can forget about pulsing anything
unless you also intend to include an inductor in
a circuit arranged as an actual dc-dc power
converter if you expect to get eff up.
There are quite a few ic's out there that do
buck conversion with somewhat small inductors
so if you're after high eff that's your best bet.
Another trick is to drive the LED with a single
series resistor and cut the current in order
to reduce battery drain. This reduces brightness
to some degree, so get an LED with a high mcd
output to compensate. If you cant stand the
variation in output over battery life, use an
LDO current regulator.
Using a linear circuit is going to be much easier
and straight forward over a true switcher, so
buy a high output LED and lower the target current
to half what you originally planned, if possible.
I think i understand you correctly, but if not
feel free to explain...i didnt notice this thread
right away so i had to quickly read through
the five pages of arguments
Also, i cant help wonder what you're using this
LED for...maybe it's back in one of the other
posts?
Take care,
Al