solution for a linear led driver

. . .

. . .

Pin-through-hole

So what is it you want to build? A linear V to LED current controller?

Does heat and efficiency count much? And what about illumination feedback, do you require repeatable light level from the LED over temperature?

Feedback around a current driven LED is required because their operating voltage changes with temperature. If your uC is taking a illumination measurement and controlling LED current, then you can use a PIC power switcher chip that contains the required RS latch for current switching, uC sets the peak current, and duty cycle to close the feedback loop.

Explain more about the application, does it have feedback, or is it open loop voltage to LED current?

That's only 1% ripple, should be able to get that with LC filtering?

Look at OnSemi app notes for NCP3063, that chip can drive a pair of 250mA LEDs direct, power switch in the 8pin chip, you add the coil and schottky diode and glue stuff. Add an LC ripple filter. Or use LTC stuff and simulate to your heart's content.

Grant.

Thanks all for your reply... with your help i've made this circuit:

1. an opamp (LT1368) with Vss=0V/Vcc=12V
2. a mosfet (IRF540) with * gate connected to opamp Vout * source connected to 20ohm resistor and opamp V- * drain connected to led cathode
3. Led anode connected to Vccled=12V
4. resistor connected to Vss=0V

If a variable voltage (between 0V and 3.3V) is generated at opamp V+ pin, i obtain a linear current regulation. BUT:

1. why with an opamp V+=0V, led is attraversed by a non zero current? Current is very small, but led emits in this condition...
2. why with a Vccled greater then 12V i obtain strange results? For example, with Vccled=24V and opamp V+=0V, led randomly flashes. If i increase V+, led current increases in a not controlled way...

@Grant

yes

no

from the LED over temperature?

I thought about current sensing with resistor described above. I thought about resistor voltage drop measured by a PIC ADC: what do you think?

I don't understand...have you any link?

I'll give a look and i'll tell you later...

@John Fields

I want to use this led for eye illumination and eye image acquisition with a camera...

Pin-through-hole

with a voltage generated by a pwm signal and a low-pass filter

i need a ripple lesser than 2/3mA

. . .

--- This seems to work pretty well:

Version 4 SHEET 1 880 680 WIRE 208 -64 -128 -64 WIRE 352 -64 288 -64 WIRE 352 -16 352 -64 WIRE 176 80 -64 80 WIRE 352 112 352 64 WIRE 176 128 176 80 WIRE 144 144 0 144 WIRE 288 160 208 160 WIRE 144 176 96 176 WIRE 96 256 96 176 WIRE 352 256 352 208 WIRE 352 256 96 256 WIRE -128 304 -128 -64 WIRE 0 304 0 144 WIRE 176 304 176 192 WIRE 352 304 352 256 WIRE -128 432 -128 384 WIRE -64 432 -64 80 WIRE -64 432 -128 432 WIRE 0 432 0 384 WIRE 0 432 -64 432 WIRE 176 432 176 384 WIRE 176 432 0 432 WIRE 352 432 352 384 WIRE 352 432 176 432 WIRE -128 496 -128 432 FLAG -128 496 0 SYMBOL Opamps\\LT1368 176 224 M180 WINDOW 3 12 7 Left 0 SYMATTR InstName U1 SYMBOL npn 288 112 R0 SYMATTR InstName Q1 SYMATTR Value FZT849 SYMBOL res 336 288 R0 SYMATTR InstName R1 SYMATTR Value 4 SYMBOL voltage 0 288 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value PULSE(0 1 0 1) SYMBOL voltage -128 288 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 24 SYMBOL voltage 176 288 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value 3.3 SYMBOL res 304 -80 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 39 SYMBOL res 336 -32 R0 SYMATTR InstName R3 SYMATTR Value 52 TEXT -24 464 Left 0 !.tran 2

The link to your LED is no longer valid, so I just substituted a resistor (R3) which drops 13V with 250mA through it for the LED.

R1 gives a drop of 1V with 250mA through it, and the reference voltage from the PWM filter varies from 0 to 1V, corresponding to 0 to 250mA through the LED.

R2 is used to reduce the power dissipated by the transistor and is chosen to drop a little less than the supply voltage minus the sum of the drops across the LED, the transistor's C-E junction, and the sensing resistor, R1, with 250mA through the string.

-- JF

Hi to all, i've made a circuit like described in a previous post:

this circuit works fine, but i have strange results:

1. LT1368 sometimes becomes very hot and is unable to drive IRF540N. I have no idea why an op-amp becomes hot: i've controlled all circut and seems correct... Another strange thing: this problems appears in randomic way...
2. I've tried to change opamp and i've used TI TLE2082: all connections are the same described above. This opamp is always unable to drive led....why?

Thanks for help

--- Here's essentially your circuit with a triangle wave input, and the opamp dissipates less than 5 mW:

Version 4 SHEET 1 880 680 WIRE 448 0 448 -16 WIRE 448 96 448 64 WIRE 176 144 176 128 WIRE 144 160 16 160 WIRE 400 176 208 176 WIRE 144 192 112 192 WIRE 176 224 176 208 WIRE 208 224 176 224 WIRE 112 272 112 192 WIRE 448 272 448 192 WIRE 448 272 112 272 WIRE 16 320 16 160 WIRE 176 320 176 224 WIRE 448 320 448 272 WIRE 16 416 16 400 WIRE 48 416 16 416 WIRE 16 448 16 416 WIRE 176 448 176 400 WIRE 176 448 16 448 WIRE 448 448 448 400 WIRE 448 448 176 448 WIRE 16 528 16 448 FLAG 16 528 0 FLAG 208 224 +24 FLAG 176 128 0V FLAG 448 -16 +24 FLAG 48 416 0V SYMBOL res 432 304 R0 SYMATTR InstName R1 SYMATTR Value 15 SYMBOL LED 432 0 R0 SYMATTR InstName D1 SYMATTR Value AOT-2015 SYMBOL voltage 16 304 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PULSE(0 3.5 0 1 1 1e-3 2) SYMATTR InstName V1 SYMBOL voltage 176 304 R0 WINDOW 0 -57 13 Left 0 WINDOW 3 -52 95 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 12 SYMBOL Opamps\\LT1368 176 240 M180 WINDOW 0 37 126 Left 0 SYMATTR InstName U1 SYMBOL nmos 400 96 R0 SYMATTR InstName M1 SYMATTR Value NTB52N10 TEXT 24 472 Left 0 !.tran 3

Perhaps your circuit is oscillating sometimes?

What does your PWM input to the opamp look like?

-- JF

Thanks for your answer...

How can i verify it?

Micro PWM is connected to a rc circuit (R=4.7k, C=1uF): in this way i obtain a "constant" voltage to opamp input. Micro PWM is 0/3.3V; a "constant" voltage at opamp +pin is present. What do you think?

Thanks for your reply...

Your last circuit is identical to mine: you have used R=100k C=10nF, i have used R=4.7K C=1uF...this could be a problem? I've choose these values so i can obtain a low voltage ripple to V+ opamp imput...what do you think?

Thanks