LED backlight driver

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.
.

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