LM2409 Analog current dimming issue 2

[CTLee123]

Hello,

On the LM3409 we are driving 4 series LEDs and are having difficulty getting the LEDs to dim far enough down using the IADJ pin (using the analog 0-1.24V control). At 0V, we still have about 13mA @ 10V flowing through the LEDs string and thus the LEDs are still lit. Looking more closely at the datasheet (

http://www.ti.com/lit/ds/symlink/lm3409.pdf),

we now see that the IADJ voltage only gets the output current close to zero, but does not actually take the output down to 0mA. What could we try to force the output current to 0mA when the input voltage is 0V? We could use another separate switch to control the enable pin, but it would take a major mechanical change for that. Another workaround would be to find a simple way to automatically shunt 13mA or so away from the LEDs, which would waste some power but would be better than lit LEDs in the "OFF" position.

Anybody's help is much appreciated!

-C

 

[IRstuff]

Why not apply a PWM signal to the EN pin to further dim the LEDs? Or turn off the LEDs altogether with the EN signal?

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[Comcokid]

I'm not familiar with the LM3409, but reading your post and briefly looking at the datasheet I would suggest adding some type of circuit on the IADJ so that when you bring IADJ real low it either acts on the EN or the UVLO pins to turn the chip off. Such a circuit might consist of a comparator, or a shunt regulator IC. I mention a shunt regulator as many of these devices consist of a low voltage reference, comparator, and sometimes uncomitted drive transistor almos ready made for this type of function/control - Diodes Inc ZXRE06 / STM TS4436 / OnSemi CAT102. What you achieve is a small circuit that functions like a mechanical switch when your adjustment is turned-down near minimun.

 

[CTLee123]

We had considered PWM originally but it would have required an extra part and this is a commercial product so we were trying to keep the part count down. It would have been simpler in hindsight, but at the time we thought the analog and PWM controls would be able to dim equivalently.

The idea of turning off the chip at 0V is one that we are probably going with, but even that requires adding in a little comparator to detect when we are near 0V on the slide pot IADJ input. The other downside to that approach is that at 13mA, the LEDs are still quite bright so we're missing the lower end of the light output scale if we just turn off at that point. It would be preferred to dim the LEDs all the way down and THEN turn off the circuit, hence looking for a way to "shift" the output of the IC down by about 13mA so that it dims all the way down on the way to turning completely off.

Thanks for the suggestions!

 

[CTLee123]

Comcokid, I will take a look at the shunt regulator. Are you saying that the shunt regulator would provide a good abrupt transition that we could use to drive the EN pin? I think we need to turn off pretty close to 0V (since we still have usable light even AT 0V), so I will take a look at the datasheet to see when we would expect the output of the shunt reg to dropout. I think I am not quite clear on how you are saying we should use the shunt regulator in the circuit.

 

[IRstuff]

Either way, you're going to need to add components. If you're looking for something that's automous, you can use a Widlar or Wilson current mirror and design it to dump exactly 13mA. A simpler, but less efficient approach is to hang a 300 ohm or so resistor in parallel.

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[CTLee123]

IRStuff - yes that's what we ended up doing after some deliberation (putting a 300Ohm resistor in parallel with the LED load). To avoid draining power when the LED is off but the voltage is still on, we will use a comparator to completely shut down the IC when we are near 0V for the IADJ pin.

Thanks for help.