Re: LED backlight driver

ehsjr wrote:
Jim Yanik wrote:

ehsjr <ehsjr@xxxxxxxxxxxxxxxx> wrote in zaPgj.5002$O97.4020@trndny01:">news:zaPgj.5002$O97.4020@trndny01:


news@xxxxxxxxxxxxxxxxx wrote:

Hi,

A new product I'm designing uses a TFT display with an LED backlight.
The LEDs are arranged as 5 series strings of (I think) 9 white LEDs.
Each string is spec'd at Vf = 30V @ 20 mA. They also recommend that
the currents in each string are matched to within 5% for good
brightness uniformity.

The power supply is a wall-wart with battery backup, giving a supply
rail of 6 to 9VDC.
The brightness needs to be adjustable down to 2 mA per string.

There are several schemes I can think of to drive this, none of them
particularly elegant:

1. Five boost converters (LT3461 or similar), one driving each LED
string, feedback pin connected to current-sensing resistor.

2. One boost converter to provide a regulated 35V rail, then a linear
current regulator (op-amp + npn or MOSFET) in the negative end of each
string.

3. Connect all 5 strings in series and drive from a 150V boost
converter. This would probably need a transformer rather than an
inductor, so we'd be looking at a custom-wound part.

This isn't a particularly cost-sensitive application, but none of the
options above look particularly great.

There was a fourth option, similar to (2) but with the strings
connected in parallel, then a single current-sense resistor connected
to the SMPS feedback pin. I don't physically have the LCD panel yet,
but I tried a few strings of Nichia NSPW300's in parallel, with 10R
current-sharing resistors, and couldn't get them to balance even
within 50%.

Anyone got any better ideas?

Thanks

Rhydian.


Not sure ... you could use the 35 V and put a current
mirror in there if you need to adjust all the strings
simultaneously for you 2 to 20 ma requirement. If you
need individual control, an LM317 with a 62 ohm R and
500 ohm pot in series between Vout and Adj will give
you about 2.2 to 20.2 ma from the adj pin.

Ed



the question is;will his LEDs output the same brightness with identical currents? I suspect there will be a significant variation between LEDs.


That's why the individual control circuit might
be preferable. OTOH, he could put a series trimming
^^^^^^
parallel
resistor in each string driven by the mirror.

Or are you referring to different brightness in
individual LEDs within a series string of 9 LEDs?
Individually controlling 45 LEDs can be done, of
course, but it may be impractical for the op.

Ed
.

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