is an output control switch of LED2001 practical

[Dsun.21]

Dear Support/Expert 

I have a question about LED2001, is it practical to apply a relay to switch the LED off, but keep the DIM enabled.  as in the schematic. if K21 is off, it will cut the current off of the output, but the DIM pin of the LED2001 is still enabled. 

will this operation cause any damage to the chip? or any negative side effect. 

Thank you and appreciate your help.

David

[Peter BENSCH]

No, that's not a good idea. Firstly, the SSR TLP3545 is already set to NRND by the manufacturer, so it will only be available for a limited time, secondly, the SSR is intended for currents of max. 1A, the LED2001 delivers up to 4A (and you also envisage 25mohms for this).

But why switch large currents with additional devices when you can use the LED2001 as a switch much more elegantly?

All you need is possibly an additional logic gate (if the PWM controller, which is not visible here, cannot do this on its own) to disable the LED2001 via pin DIM = Low.

Hope that helps?

Good luck!
/Peter

[Dsun.21]

Hi Peter, 

Thank you very much for your prompt reply. we will replace the SSR for a new one with higher current. the reason we need SSRs is that we have several LED loads will be selected exclusively. all my concern is that if we use the SSR to cut the current of the output, will that have negativity effect (damage the chip for long term) on the LED driver?

Best Regards,

David 

[Peter BENSCH]

Despite the larger SSR, it is not really a good idea to switch off the LEDs behind a buck controller for power LEDs without communicating via the pin FB that everything is OK with the current.

Let's look at the following scenario:

• the PWM signal at pin DIM is logic high, which enables the LED2001
• the LED2001 wants to fulfil its task and starts switching until the voltage at FB reaches the setpoint value, which corresponds to the preset LED current
• you interrupt the LED current with the SSR, which causes the voltage at FB to drop to zero
• this signals to the LED2001 that no more current is flowing and therefore it pumps more energy into the coil, which in turn causes the voltage at C81 to rise
• the voltage at C81 increases due to the lack of feedback to such an extent that either C81 bursts or (more likely) the SSR gets "opened" again (bursts)

Poor LED2001 only wanted to restore the preset current...

If you want to argue that you are only switching the LEDs: the LED2001 operates at 850kHz, i.e. a period of approx. 1.8µs. Switching from one LED channel to the next does not take place within these 1.8µs, but can take a few milliseconds with the TLP3545A, as with all optically operating SSRs.

Summary: I'd recommend to forget the idea of switching the LEDs. As the TLP3545A is roughly at the same price level as an LED2001, there should be no reason not to use a dedicated LED2001 for each LED channel. You should just check to what extent the additional costs of the respective coil and capacitor play a role.