> >-  Drive LEDS with up to 350 mA total consumption at any one time.
>
> Drive 3 leds at 550ma each. Normal is 350ma but these can be
> over driven at 550 when pulsed.
>
> >- LEDs operate OK with <= 2.5V drop across them.
>
> 2.95 (range is 2.3 to 3.3)
>
> >     ie are these LED strings or single LEDS?
>
> These are 3 single leds (big ones...)
>
> >     What is the voltage across the LEDs when running at full desired
> >current.
>
> 3.0v  (but these will operate down to 2.31)

If you make the mods I suggested before you will get a working but not ideal
solution which can then be refined.

You are going to have to look carefully at current versus voltage across the
LEDs, what level you MUST have for this voltage, what the variations are
between LEDs and with temperature. The problem is that the voltage available
is VERY close to what you need. The normal practice is to have current
limiting resistors which are large enough to make the supply used 'look"
somewhat like a current source. You do not have that luxury here. Knowing
the exact LED type and characteristics and their spread and variation in
operation is essential. That said, something that works "OK" will be easy
enough to do (probably).

What is the LED part number(s)?
Where can an on web datasheet be found.

An active current source may be desirable. Whether this requirement is more
annoying than reworking the board to 5 volts is something you will need to
determine. Simple current sources using a transistor require drops of at
least 0.6 volts plus whatever is dropped across the controlling resistor.
Losing 0.6v is not an option here. Current sources using 1 op-amp and a
transistor will work well here. The opamp is cheap and compact but may be
annoying to add depending what stage the rest of the design is at.

A very useful set of questions is - "

- What current will the display draw if connected directly to 3.3v,
- Is the brightness and current draw acceptable,
- Is this outside the display's specs"
- What is the maximum rated OPERATING voltage for the display.



        Russell McMahon
        Best address is          Rusl_Mc@paradise.net.nz

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