Is it safe to operate the STCS1A LED driver with the EN (shutdown) and PWM pins tied together?

[jfathman]

I am developing software for an existing board that includes the STCS1A 1.5 A max constant current LED driver.

Our board currently uses that chip with the PWM pin set to steady on or off only, and it works fine. We have many in production.

Now we are considering sending an actual PWM signal to the PWM pin instead of a steady on or off.

Our existing board has the EN (shutdown) and PWM pins tied together on the STCS1A. That seems reasonable for simple on/off, but may not be suitable where a rapid PWM clock is tied to the EN pin.

When we try it, with a PWM signal tied to the ganged EN and PWM inputs, it works fine. LED power ramps up and down just as you would expect with varying PWM duty cycle.

So our question is whether it is safe in the long run to operate the STCS1A with the EN and PWM pins tied together, where EN (shutdown) clocks the STCS1A on and off at the rapid PWM rate.

Thank you.

[TSu.1]

not sure about STCS1A but I tried with STCS1PUR it almost fried my MCU. VCC somehow leeched over to the EN / PWM pin and sent 10V into the MCU GPIO.

[jfathman]

Thank you, TSu. Your STCS1 is pretty close to my STCS1A. The difference seems to be 'Slope control with external cap' has been added to the STCS1A.

From your other post, you appear to be running at 12V. For what it's worth, we are running at 5 V.

We also have an NPM switching transistor MMBT2222A between our MCU PWM pin and the STCS1A ganged EN and PWM pins. So that isolation may be protecting our MCU from the 10V (from 12V) you experienced.

I am hoping someone from ST who knows how this chip works internally will weigh in and say there is no harm in toggling the EN pin at the PWM rate.