[moved] LED control with PWM and Constant current source

Hi Everyone,

I am using MCU C8051F972-A-GM in my project . I have 6 LEDs in my board connected to the GPIO pins of the MCU. I want to design a circuit for constant current control with supply current in uA(when all LEDs are OFF) and with PWM dimming. I searched for the Constant current LED drivers with PWM control, but all seems to have higher supply current. Can anyone suggest alternate approach for this?

karg3 said:

I have 6 LEDs in my board connected to the GPIO pins of the MCU.
I want to design a circuit for constant current control with supply current in uA(when all LEDs are OFF)

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The question appears to be incorrect, or perhaps unclear. Are you meaning you want to control the driven current of 6 LEDs in steps of uA's ?

Yes, question is a bit unclear.

The µA specification is about circuit quiescent current. Unclear is a least how the "pwm" dimming shall be configured.

The usual solution is to use transistor switches (BJT or MOSFET) and current limiting resistors and generate the pwm inside the processor (least part count). If you rely on constant current you can supplement the transistor drivers to form true constant current sources.

There may be low standby power LED controllers available, but I'm not aware of specific types. The NXP I2C LED drivers e.g. have it, but no CC outputs.

The problem with current controlling LEDs is that there is a very non linear relationship between current and light output.

You will get much better results by keeping the current constant, set at the maximum desired brightness level, and switching that full current on and off with PWM.
This is particularly effective with seven segment displays and where you need good matching of brightness between segments that may have very poor brightness matching at very low current.

Nothing looks worse than a mosaic of multiple LEDs that are run on much reduced current, where the whole thing looks very uneven and patchy.

The same mosaic run at full current, but very small duty cycle will dim down wonderfully even right down to completely off, even with poorly matched LEDs.

HI All,

Below is my schematics , I have 3 dual LEDs drived by the MCU GPIO. There is a separate pwm control added using FET logic.
The problem is the brightness of the LED is not sufficient and has to be increased. So as of now I am changing the R value to increase the current since the brightness of the LED depends on the current. I thought using constant current Led driver with PWM to current regulation. When using LED driver, there is a problem of standby current which is from 3 mA to 10mA max,the MCU GPIO cannot source that much during sleep state. SO I want a LED driver that has supply current(when all LEDs are OFF) in uA. Else can anyone propose an alternate way to solve this?