Other Parts Discussed in Thread: LM3409
Were using the LM3409HV with a shunt FET. Our circuit is very similar to the circuit in AN-1953 LM3409HV Evaluation Board (see below). Five of these drives a bank of LEDs.
The problem is that when the driver should be "off", i.e the PWM signal is low, the controller is still switching, generating a 0.7% duty cycle 2.9 to 3.1 kHz signal to the switching FET (Q1). This creates a lot of piezoacoustic noise in the capacitors, especially when there are 5 LED drivers close together. And the ear is very sensitive to 3 kHz. We can't ship a product this noisy so I have to get rid of the sound.
When the PWM signal is held low, the EN pin is at a steady 5 volt. Is this what causes the LM3409 to keep switching, and could you suggest a way to stop the LM3409 switching when PWM is held low?