I am trying to use the STCS05DR constant current LED driver IC by STMicro in my buck converter design to indicate input power with a 1206 red LED.
The input power is a variable DC battery source (Milwaukee M18 battery) up to 20V. The reason I went this route is obviously to keep the brightness of the LED relatively unchanged with the gradually falling input voltage over time.
I have had no success using this IC and am wondering what am I missing to utilize this correctly.
The first time I plugged the circuit in I ended up releasing the magic smoke from the IC, the LED lit up very briefly beforehand.
Upon troubleshooting, I noticed that pin 6 which is GND was disconnected, which is strange because I imported the KiCad symbol and footprint directly from SnapEDA, therefore I was surprised to find GND unconnected by default. I scraped away some of the solder mask on the ground plane in front of the pin and solder jumped the pin the plane to connect it.
After replacing the burnt up chip with a new one and also populating a new LED, the second time I plugged the circuit in the LED fried but the IC stayed working (I think).
When measuring the open circuit voltage across the LED I got 19ish volts, which seems like a lot that would make sense why it would fry the LED. I have the current set in my use case to 20mA with a 5 ohm resistor across FB and GND, which according to the datasheet is the correct way to set the current limit.
PWM I have left floating which I wouldn't think would have an affect on anything.
Do I still need a resistor on the output to drop the voltage? Wouldn't this defeat the purpose of the IC? I also don't have a CDRAIN capacitor strapped across DRAIN and FB resistor such as the datasheet application suggests.