piclist 2002\09\06\133123a >
Thread: [EE:] Driving a 15W LED?
www.piclist.com/techref/microchip/devices.htm?key=pic
picon face BY : Harold M Hallikainen email (remove spam text)



    I've got an application where we need to drive a high power LED.
It's rated at 700mA at about 6.5V, but with adequate heat sinking, it's
been run up to about 2A with 7V across it. Back when we were looking at
700mA, I was considering the LT1618 boost converter with a 4.8V battery.
But, now that they want the higher current, the switch in this part can't
handle it. Also, we're now looking at moving from 4 nickle metal hydride
batteries down to a single lithium ion, reducing the voltage from 4.8 to
3.7. The great thing about the LT1618 is that it has a current sense
amplifier in it so it can regulate based on load current instead of load
voltage, which is what we want here. I have not seen any other switchers
with a current regulator like this. Anyone know of one?
       Any thoughts on using the PIC (which will also be in the circuit) for
regulating the LED current? Since the LED can float, I could put a small
current sense resistor between the bottom of the LED and ground and
perhaps have an analog input on the PIC sense the current. The LT part
has an oscillator that turns on the switch every 750ns or so. A current
sensor turns the switch off when the switch current (and inductor
current) hits some limit. This limit is adjusted to produce the desired
load voltage or load current with there being an absolute limit on the
switch current (about 2A). It seems that the PIC that's handling the rest
of the project could also perhaps measure the switch current and the load
current and drive the external FET switch to do something like this.
However, it seems that I'd need a couple external amplifiers so the
current sense resistors could be small. My parts count is going up again!
       If I go with a standard boost converter chip (or the clever two
transistor circuit recently posted), I'd need to convert the load current
sample to a feedback voltage to drive the converter chip. Again, parts
count goes up. But, I may have no other choice.
       Any ideas out there? What'd be the simplest way to get a regulated 2A
through a 7V LED from a 3.7V battery?

Thanks!

Harold



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