LM2409 Analog current dimming issue
LM2409 Analog current dimming issue
(OP)
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
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





RE: LM2409 Analog current dimming issue
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RE: LM2409 Analog current dimming issue
RE: LM2409 Analog current dimming issue
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!
RE: LM2409 Analog current dimming issue
RE: LM2409 Analog current dimming issue
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RE: LM2409 Analog current dimming issue
Thanks for help.