Constant current controller

[Mosaic]

Hi:

I want to drive a string of 6 Piranha type LEDs, (2.1Vfwd) but my source voltage varies from around 11.5 thru 14.5V. Max continuous current is about 30mA each.

Placing a resistor alone inline to limit the current will cause brightness changes as the drive voltage changes.

Can I approach this by trying to place a FET in linear mode and control it via PWM and a gate RC DAC? I can sample the Vfwd drop across the FET to provide the ADC feedback for the PWM.

Perhaps there's a better approach?

thanks!!

 

[Jon Wilder]

Perhaps there's a better approach?

thanks!!

Perhaps a voltage regulator? Maybe? Just a thought...

 

[Mosaic]

Hmm... I guess an LDO might be an easy way out here.! I'd still be able to use the MCU to do LED string sequencing drives etc. Just thought I could double duty the MCU.

I was harboring thoughts of using the MCU to charge pump a capacitor to 'regulate' voltage since the current drain is only 30mA.

 

[Jon Wilder]

You can still drive the LEDs with the MCU. You'd just be powering them from a requlated supply. If you plan to power the LEDs from a supply that's separate from the MCU supply you'll need a transistor to buffer the LED from the MCU. But the MCU would drive the base of the transistor while acting as a current sink for the LED off of its supply.

 

[MrAl]

Hi:

I want to drive a string of 6 Piranha type LEDs, (2.1Vfwd) but my source voltage varies from around 11.5 thru 14.5V. Max continuous current is about 30mA each.

Placing a resistor alone inline to limit the current will cause brightness changes as the drive voltage changes.

Can I approach this by trying to place a FET in linear mode and control it via PWM and a gate RC DAC? I can sample the Vfwd drop across the FET to provide the ADC feedback for the PWM.

Perhaps there's a better approach?

thanks!!

Hello there,


Well, there are some alternatives you might consider.

If you break the string of 6 into two sets of 3 LEDs and power each set with its own resistor of 270 ohms each, you'll see 30ma at high input and 19ma at low input roughly. That might do it.
If that's not good enough, if you break the string into 3 sets of 2 LEDs and power each set with its own resistor of 340 ohms you'll see 30ma at high input and 21ma at low input, which would probably be good enough if you dont require super accurate light outputs.
If that's not good enough, you can break the string into 6 sets of 1 LED for each set and power each LED with it's own resistor of 420 ohms you'll see 30ma at high input and 23ma at low input, not too bad at all.

All of the above solutions will use more power than 6 in series, but you can not power 6 in series with 11.5v anyway.
Also, all of the resistor above would have to be rated for at least 1/2 watt as 1/4 watt wont be enough.