solution for a linear led driver

If you can't an IC to do what you want, then back up a bit. Perhaps a transistor would do the job.

(Can you cut down the 24V number a bit? How much voltage does it take to turn on your two LEDs in series?)

George H.

TI has tons of them that do the job- not surprisingly they're actually called LED drivers.

Ok.

Ok.

Ok. What's the problem? You have *plenty* of headroom.

Linear and PWM aren't generally used in the same sentence. ;-) Please explain. If all you need is PWM, why not just drive a FET or even BJT? Why

Depends on what is "simple" for you. Is a SOT-23-5 or SC-70 "simple"?

This is the led i shoud drive

se

=A0Why

I want to dimmer led with a micro...i need these two features

1. variable linear led current
2. led current should be dimmered by a micro output pin

24V is my power voltage

PTH components

This is what i think i can use a digital mosfet (like 2N700) to drive led current. Its gate is driven by a PWM with an RC filter: in this way i have an anolog output current. In this way i obtain a linear output led current with a pwm input signal: what do you think? Any suggestion?

ease

T? =A0Why

You don't have enough headroom- the LEDs drop 13V at 250mA making it

MOSFETs exhibit a square law relation between gate control voltage, in excess of the turn-on threshold, and drain current, so this idea is fundamentally nonlinear.

By "linear", do you mean that it's constant; not PWM switched? Why? What's wrong with PWMing the LED directly?

Your micro runs off 24V? Must be some old technology. ;-)

Ick. They still make those? I doubt you're going to find any LED drivers in PTH.

Not going to work. You're counting on the device parameters to set the current. These will be all over the place and not linear at all. Even with feedback this is going to be a mess. Note that you're going to be dissipating a *lot* of power by controlling the current with a linear pass element from

PWM directly.

Thanks all for your reply....

@krw

=A0What's

I need a linear led current. I need to use PWM only to drive linear led current

Power voltage of my board is 24V: i use this voltage to drive led. My micro works at 5V: it is obtained from a linear regulator. However, 5V is now become an old technology :-)

ivers in

should use a breadboard

Unfortunately, mosfet is used as current source..so its Id/Vgs is quadratic and not linear

Ok

om

Diode have a voltage drop of 13V (at 250mA, as you can see), so Vds should be 24-13=3D11V (as maximum value, if i not consider resistor voltage drop)

If i want a linear led current, how can i use PWM directly?

@Fred Bloggs

I know...have you a better solution?

in

Add an inductor to the PWM, you want 250mA from 24V into 13V? That's easy, a little 8pin IC is all it takes, plus the inductor, a schottky diode and some passives. There's lots of small switcher chips to choose from, and current ripple can be made low with additional LC filter. Why go linear?

Lots of extra heat to get rid of, for what additional end use benefit?

Grant.

Or, put an opamp around a FET for current source, mad but not impossible.

Grant.

Thanks for your answer...

Current ripple could be a problem for me....anyway, have you a link? i need a very simple circuit all done in pth package...

have you a schematic?

You have two main issues to be resolved here:

1. not enough voltage to drive two LEDs in series to 250mA with 13V drop each

and...

1. converting a PWM to current control with the same scale at both 3.3 and 5 V micro operation.

Hi, thanks for your answer...

Ok

this is my big problem

Do you care about wasting power? You could always low pass filter the PWM and turn it into a voltage, then an opamp, pass transitor and current sense resistor. Can the LED's float a few volts off of ground?

George H.

Thanks for your reply...

I can use your idea also with 0/3.3V PWM?

of course

Boost converter for series, or run them in parallel on linear, not a problem, each with own pass transistor, spread the heat load too, maybe a dual opamp driving twin MOSFET current sinks?

Huh? Duty cycle PWM not going to change with voltage if you servo the system properly. Output current vs duty cycle, not to an RC hang off PWM output. Anyway, uC has ADC? Current sense from LED via 24V is independent variable to what voltage uC is running. Surely you not going to run this open loop? Or if so, duty cycle a fixed reference like a TL431 2.5V.

Grant.

. . .

. . .

If you can, why not tell us something about your application, what you mean by "current ripple", and why it might be a problem for you.

Right- just so he knows the final circuit will not be simple, it will be bunches of parts: some kind Vref, an error amp, two power elements with heat sinks maybe, maybe some power resistors, not sure about the LEDs- may be better to have at least one lead soldered into a big heat dissipation plane like ground. If he boosts the supply then even more stuff.

The LED power shouldn't be any different, whether it's driven from a PWM or linear source (given the same visual intensity). Other than that, I'd want to see some major justification before going to all that trouble on a dumb LED. Perhaps it's for video illumination, or some such.

For lower voltage pulses you have to adjust the gain somewhere, longer pulse lengths or more gain.

George H.

(It's hardly 'my idea'.)

Thanks all for your reply...

@Grant

I want to use a sense resitor with led: its voltage drop is used in micro internal ADC. Linear output current vs Duty cycle shoud be the best, but i have no idea how to do it in a simple way: so i thought linear output curret vs voltage obtained by a PWM and RC filter

@John Fields

I want to use these led for eye illumination: i want a current as linear as possible. If you know a very simple led driver PTH/low components/250mA/very low ripple (lesser 2/3mA)/PWM modulated will be very appreciated. For me, power dissipation isn't a great problem: so i looked for a mosfet solution

@George Herold

Have you a schematic? An inspiration?