Current to PWM:high power LEDs on low power driver

Mods: please move to Hardware/Interfacing, I guess it fits better there! Thanks!

Hi!

I want to drive high power LEDs (18 LEDs, up to 350mA each) using a driver chip that can only sink up to 20mA per channel. My thought: A circuit/IC that linearly converts the current/voltage on each channel into a PWM signal, which can then drive a transistor controlling the LED, as follows:

Is this the best approach? What other possibility do I have to be able to sink 350mA from a driver that sinks 20mA? While preserving the proportionality? (i.e. sinking 10mA at the driver means sinking 175mA at the LED) Feel free to fill out the magic cloud below :)

Requirements: Circuit must SINK current, because the LEDs are common-anode RGBs. Assume there is no alternative for the driver chip, unless you know an I2C compatible PWM driver with at least 18 channels!

Any hints and suggestions appreciated!!

Sorry forget it you are barking up the wrong tree. If your LED draws 350 mA you need a constant current source to drive it not a puny 20mA driver.
See this discussion:-
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1234273497