Driving 4 white LEDs independently

Question

I'm trying to design an efficient circuit that is able to drive 4 independent (ie, connected in parallel) white LEDs and to control their intensity independently as well, through PWM. The thing is: most of the circuits I find were designed either for 3 "strings" (rows of LEDs), which is cool if you use RGB LEDs, or don't provide a way to control the intensity of each string.

Using multiple driver ICs, each one controlling a single row, would destroy the efficiency of the whole circuit.

The white LED I'm using: http://www.mouser.com/ds/2/311/LW%20Y1SG%20-%20Micro%20SIDELED-335725.pdf Forward voltage (max): 3.6V Current: 30mA

Also, two other requirements are: it should have low quiescent current (ideally < 1uA) and Vin(min) > 1.6V since the circuit may be powered by two alkaline batteries.

This is the first time I'm designing such circuit, so I'm very looking forward to hear some tips from those who have done this before. Thanks in advance!

Answer 1

You could do a sort of modified high-side PWM controlled joule thief for each of the LEDs, with each having an independent PWM controlling the power though the LED.

Simple version might look something like this...

^{simulate this circuit – Schematic created using CircuitLab}

This is different from a normal joule thief in that...

1. The chopping is done via PWM rather than feedback. This lets you control the power though the LED.

2. The LED is connected to the high side rather than ground. This means you will need more kick in the inductor to get the LED up to minimum voltage, but has the advantage that there is no leakage current though the LED when the PWM is off.

Note that there is not a straight relationship between PWM and brightness here. When PWM is off, LED is off and no current flows. As you increase PWM duty cycle, you will hit a minimum current needed to light the LED. Then as you increase more, you will get brighter and brighter LED until you hit a maximum. Past the maximum the LED will start getting dimmer and you will start killing the battery, so stop increasing the PWM duty cycle when you get to the maximum brightness.

This circuit should have very low quiescent current- basically whatever the leakage current of the transistor is.

Answer 2

Here is my idea. Consider this a sketch or snippet.

^{simulate this circuit – Schematic created using CircuitLab}

VCC_LED should be the output of a boost regulator that produces something like 4V. L1, C1, D1 and M1 are meant to represent the boost regulator. Values are just guesses.

D2, Q1, R1, D3 and D4 are the current mode drive for ONE LED. That will have to be duplicated 3 more times for the rest of your LED's. D3 and D4 could be combined in a single small package. To set the current, adjust R1. Basically, one of the diodes approximately cancels out Vbe of Q1. So the voltage across R1 will be around 1 diode drop. Q1 will be in the linear, high beta region. This is not a saturated switching application.

I am pretty sure this will work with a minimum of fine-tuning. I have used circuits like this before in mass production. If PWM1 was from a regulated supply, D3 and D4 could be replaced with a single resistor. That is why I asked about that.