LED Dimming via PWM and/or Current control

JimmyM

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Aug 30, 2006
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Boston, MA, USA
I'm working on an idea for a high power RGB fader for a project.
I have a few things to work out regarding LED fading. Constant current/limited current is good for driving LED near their peak since it avoids thermal runaway.
But LED brightness can be unpredictable/unstable at very low drive levels. PWM is good for that.
I've recently built a microcontroller based constant current regulator whose output current is controlled via a pot for the moment. It uses a 250kHz switched mode output stage to drive an LED from 0-~350 mA. It can drive several in parallel at >900mA. But at very low current levels, the LED brightness is a little wonky and no where near constant.
My idea is this. From 0 to some middle brightness, have the regulator maintain a constant voltage and use PWM to vary the brightness from 0% duty to 100% duty. Once the output reaches 100% duty, switch to current mode and monitor and control the current to increase brightness to it's maximum of ~300-350mA.


Have you LED guys heard of this type of approach before?
 

georges80

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Oct 23, 2002
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Sunnyvale, CA
Yes, current regulation at LOW currents is difficult since typically you have a small value sense resistor that is being monitored to set the constant current. At low currents the voltage being sensed is VERY small and any noise in the system will cause the switcher control loop to hunt and therefore you get flicker in the LED or inconsistent current drive or even total lack of switcher startup. Some LEDs won't even light at low current levels (e.g. Luminus LEDs).

So, what I've done on several of my LED drivers is use constant current down to a certain drive level and then PWM below that. e.g. on the h6flex I will provide constant current regulation from 6.7A down to 1A and for dimming below 1A I will use PWM of the 1A signal. e.g. for 50mA drive I will use a 1:20 duty cycle of 1A constant current.

Some LED switcher driver IC's (Linear Tech has a whole range) have both constant current output adjustment via an analog input voltage AND also a PWM control input. You can then use a combination of the two control inputs to provide the necessary dimming range and peak current output.

cheers,
george.
 

JimmyM

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Aug 30, 2006
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Location
Boston, MA, USA
Thanks, George. I was kinda hoping to get your input in here. I'll have another look at those linear chips.
 

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