High power led driver adjustable voltage

[Murugesh_89]

Hi,

I want to adjust the brightness of a 8W LED ( 12 V, 0.7 A ) for the "scale" of 0%( OFF ) to 100%( Full brightness ) from my microcontroller. I will get the "Scale Input" from a HMI.

I dont want to design it using the PWM method.

Please help on designing the " variable analog high power led driven circuit ".

Any schematics, any web link is really appreciable...


Thanks,
Murugesh

 

[d123]

Hi,

I don't know of any links, but could you implement a voltage to current block (e.g. op amp + BJT) between MCU and LED? Or use something like the XTR110 Precision Voltage-to-Current Converter/Transmitter could be adapted for this purpose, increasing the base current to the BJT based on a scale you write in code ranging from off to fully on, if you can adapt an output signal (voltage) to do more than 0/1 on the MCU.

 

[KlausST]

Hi,

LEDs should be current driven, not voltage driven.

Therefore you need to look for an adjustable current source.
There is an almost perfect relationship between current and brightness.

Klaus

 

[Murugesh_89]

Therefore you need to look for an adjustable current source.

Klaus

Then how to design a 12V variable current ( 0 - 700 mA ) driving circuit.. any ideas or suggestions please..

 

[KlausST]

Hi,

Define the HMI signals for 0% brightness and 100% brightness.

Do you want linear regulation or a switch mode regulation.

Klaus

 

[betwixt]

Your MCU almost certainly doesn't have an analog output so you have to create one somehow. You can do it with a DAC, possibly one made from nothing more than a chain of resistors or an external DAC IC. Whichever way you do it, you MUST generate an analog voltage to control an analog circuit. From there it is simple, follow what Klaus advises, make a controlled current source and add a resistor in series with the LED. The voltage dropped across the resistor is proportional to the current through it so you can use an op-amp to compare it to the DAC voltage to set the current.

Much easier with PWM if you can be persuaded to use it!

Brian.

 

[Murugesh_89]

Well I am aware of PWM method. But the issue is I will have to send this for EMI/Emc testing. PWM method will switch LED much quickly, also it is quite high power LED. So I am worrying about the amount of Radiation it will emit.

 

[betwixt]

suggestion:
as you are going to have to generate an analog voltage to control the LED current, use PWM to generate that voltage rather than switch the LED directly. In other words, use the PWM as a DAC. You might be able to get away with just a simple RC filter to convert the PWM to a DC voltage. It uses far fewer components than a discrete or IC DAC design and gives you finer control of the LED current.

Brian.