Hi,

I want to drive a strip of 15 650 nm LDs (3V, 40ma) with 48V switching power supply. I intend to use constant current LED diver NSI45035JZ. https://www.onsemi.com/pub/Collateral/NSI45035JZ-D.PDF. I’ve got the driver but my scope has a pretty low bandwidth( 0- 200kHz) so I would not be able to see a turn on voltage spikes if there are any. Will it be safe to put 0.1 µF electrolytic capacitor in parallel with the load to account for potential spikes or I need more sophisticated circuit?

 

Why are you concerned about voltage spikes with a constant current source?

 

Why are you concerned about voltage spikes with a constant current source?

Soft turn-on, is recommended for laser diode drivers to avoid damage from turn on voltage spike. Also in powering down, the laser driver should protect against excessive voltage; and , it responds to an open-circuit condition by shutting down the laser before the circuit can be closed, which could allow a massive current spike.

 

With 45V across the string of LEDs you only have a voltage overhead of 3V to allow the driver to function – so you might be sailing close to the wind in terms of maintaining a constant current drive. Figure 1 from the data sheet suggests you need at least 5V (anode to cathode).

The main reason to ensure that an excessive voltage is not applied to an LED under a fault condition is that the laser light output might become hazardous – especially if the beam can be viewed directly.

With one of the LEDs in the string failing short circuit, the driver would maintain the constant current and should the driver fail short circuit, the string of LEDs would have 48V applied across them. Check the LED data sheet to determine if this condition is within safe voltage limits.