Hi

I am working on a design that will be using LED from Cree. CLQ6B_TKW-2887154.pdf (mouser.co.uk)

Previously this was my design, as i was using a 12V.



I will now changing the power supply from 12V to 5V, therefore my resistors are also reduce to about 30 ohms.

Previously i was using PWM a mosfet, i am now thinking about using a standard 2N3904 transistors , as i am using this else where in my design. And as the max current for the LED is set to 100mA.

Is this the correct approach to use PWM on a LED, instead of using a driver IC?

The PWM will be outputed from the microcontroller.

Thanks

 

Is this the correct approach to use PWM on a LED, instead of using a driver IC?

Either way is acceptable as long as you stay within the specs of the components. Personally I would use the mosfets and keep the design as is.
LED's have different forward voltage drops depending on the color, so probably need to adjust the values of the current limiting resistors.
See page 2 on the spec sheet. TYPICAL ELECTRICAL & OPTICAL CHARACTERISTICS .

 

Any particular reason to keep the MOSFET? I have option but to drop the 12V as i am working with 5V now, so looking to see what other changes can be made.

At moment, I have kept all the resistor the same, but I as per the forward voltage this can be further optimised.

 

MOSFETs will be lower loss. Rds(on) = 24mOhm so at 0.1A losses are 0.1x 0.1 x 0.024 = 240uW

2N3904, Vce(sat) at 100mA is ~0.3v , Vbe(sat) is ~0.9v @Ib=10mA Losses are 0.3 X 0.1 + 0.9*.01 = 39mW

In reality this makes little difference...

On the other hand, 2N3904 1000up are about 1/2 the price of the MOSFET and save you 4 resistors (R17-20 no longer needed), but then are through-hole so potentially increased assembly cost (why are you not usng smt part? or are you using smt equivalent?)

 

Thanks.

The other reason for using 2N3904 , is that i got few reels of them so would like to use them up.

Sorry the full part number is MMBT2N3904,which is SMD version.

 

What's nice about the mosfets is if you decide to change the LEDs or supply voltage you won't have to change the bias resistors.

 

Here's an interesting thought--
Since it sounds like you have plenty of transistors, how about using two transistors to do double-duty as both a constant-current source and a switch to control each LED?
That makes the LED current essentially independent of the supply voltage, and eliminates the series current-limit resistor (of course that's replaced with the current determining resistor).
Below is a LTspice simulation of such a configuration:

Note the the LED current (green trace) reaches its constant current value of 100mA at about 3.8V input (red trace) for a 3V LED drop, and stays constant as the input goes to 12V and back to 5V.

The LED shuts off when the 3.3V control voltage (blue trace) goes to 0V.

 

Here's an interesting thought--
Since it sounds like you have plenty of transistors, how about using two transistors to do double-duty as both a constant-current source and a switch to control each LED?

@crutschow That's an excellent suggestion!