Other Parts Discussed in Thread: LM3409, PMP15004, TPS92515
I have been struggling with this for a bit and wanted to get this question out here. I see a lot of information in these forums, so I am eager to see what is said.
I have a circuit with 3 LM3409 drivers, but this issue is present with even one driver. There is a ripple voltage from 360mV to 420mV present on my Main power rail. This ripple is at the same frequency as my PWM switch controlling the current to the LED. I am trying to reduce the ripple with input capacitance but I am up to 440uF and now inrush current is becoming a concern. I have researched other causes and ringing is a possibility. I have added a snubber to the Mosfet (D to S). This did eliminate the ringing and improved ripple from my first prototype.
1) Since this is a P Channel switch, should I still use the Snubber on the Mosfet, OR should it be placed to the ground?
2) I saw this "slew rate control for EMI" article:
I am wondering if I can use CSP and CSN with a series resistor to control slew rate and eliminate any other type of ring on the switch? I wanted to ask here before investing my time in a redesign.
3) Are there other ways to combat this like isolating and filtering the three drivers from the other circuit. The rest of the PCB is just linear control and MCU that is controlling the PWM switch.
Mosfet used: SQS401EN-T1_GE3CT-ND
Inductor used: SRN8040TA-220M
Switching Frequency of IC: 500KHz
Switching frequency of PWM: 20KHz
47uF Output Capacitance