LEDs circuit control via PWM from PIC12f675: calculate base resistor for an N-channel MOSFET

Question

I want to "develop" a project with some bi-color LEDs (OSRAM LSY-T676.) I have made (after some tutorials) my own component because it was not included in Proteus 8 library. Source which is used is a car battery (12V), the LEDs have a forward voltage of 2V for both colors and a forward current of 20mA (nominal.) I need to use twenty of these LEDs, so I made four groups of five LEDs (10V forward voltage.) After basic calculation, a 100ohm resistor is needed to obtain the correct current.

I'm want to control the LEDs with a PIC12F675 microcontroller (as I read in datasheet: current 25mA and Vdd-0.7V => 5-0.7= 4.3V output if Vdd will be 5V as I want.) The idea was to use a 2N7000 N-channel MOSFET (Vgs(th) =2.1V Id=115mA) but I'm not really sure if I interpreted the data for the microcontroller and MOSFET correctly. Could you, please, correct me, if somethingbis wrong? My questions are:

1. What is the maximum frequency for PWM (I want to control LEDs via PWM.)
2. How should I calculate the base resistor?
3. Do I need a parallel capacitor? (I saw some examples on forums.) If it is needed, how can I calculate the value?
4. What is the influence of RDS(on)? (As I understood is one of the most important characteristic)

Answer 1

You control the MOSFET with voltage, so you do not need a resistor for the gate. The 5V from the I/O-pin are going directly to the Gate and switch the MOSFET on.

If you want to use two MOSFETs in parallel to reduce the Rds_on, then you should add 10 ohm resistor between the gate and the I/O-pin.

You should measure the voltage drop over the LED at the maximum current and then you can calculate the resistor value. It is better to use the lowest current that works for you. The 30mA is the absolute maximum value. If you use a lower current, then the endurance of this LED is longer.

Inverted driver for the MOSFET:

For stability of the voltage, it is useful to use a 12V low drop voltage regulator.

If the car should drive and the battery is connected to the car, then you need something for the safety. You have high voltage peaks in a car, negative voltage and a lot of noise. You need protection diodes and a filter.

Answer 2

You need to take into account that the car battery voltage can vary from 13.8 volts (fully charged) to about 11 volts (mostly depleted). So to be sure that the current stays below 20 mA, a resistor of 3.8/20 = 190 ohms would be needed. A better way to control LED current might be a circuit as follows:

^{simulate this circuit – Schematic created using CircuitLab}

The voltage ramp (pwl) could be replaced with a variable duty cycle PWM, and a voltage divider could be added to the base drive such that a 5V PWM will drive a maximum of 20-25 mA at 100% duty cycle. The LED current is not quite a linear function of the drive voltage, but you could monitor the voltage on R1 and adjust as needed. An op-amp could be used for a very linear I/R LED function.

(edit) The PWL (piece-wise linear) voltage source in this case is a linear ramp starting at 600 mV and ending with 1 V at 300 ms. A PWM signal will also work, with peak current limited by R1 and the Vbe of Q1.

A MOSFET should also work, with the ramp starting at Vgs(thresh), so the starting point will be higher. Stability should be OK with either device. There will be a temperature coefficient in both cases.

Using PWM will probably be most stable, although the peak current will vary. You could also add an inductor in series with the LED string, and a flyback diode connected in anti-parallel. This can essentially eliminate ripple inherent with PWM. The current limiter circuit using Q1 is optional, but might protect against destructive overcurrent in case of duty cycle too high.