LM2675: Excessive noise on output of LM2675 3.3V SOIC8 package

Hello TI,

I’m experiencing major output noise on the LM2675M-3.3 regulator. 

Specifications:

• Voltage input: 20-25V (typically 24V)
• Voltage output: 3.3V
• Load current: 0.1mA – 606mA
• The input of the LM2675 is supplied by a 400V to 24V quarter brick DC DC converter.

Schematic:

• BC_PWR also has two more 22µF adjacent to its output.
• C13-C16 = Ceramic
• D3 = Diodes Incorporated #B130-13-F
• L2 = Bourns #SRR6038-470Y
• C18 = AVX #TPSB107M010R0400 Tantalum
• C19 and C20 = Ceramic

There is some noise at BC_PWR, but the LM2675-3.3 seems to be majorly amplifying this noise.

Oscilloscope trace: No Load

• Pink – Input voltage (24V RMS, 21-26Vpp, pin 7)
• Blue – CB net (voltage on boost capacitor, pin 1)
• Yellow – Vsw (pin 8)
• Green – +3V3 (pin 4)

As you can see, the output voltage is 3.3V RMS, but swings between 2.5-3.8V.  It appears to be greatly amplifying the input noise (which the rest of system can tolerate) rather than filtering it.

I modeled the system with the closest available components in webench, and the system appeared stable. 

1. Do you have any suggestions?
2. What could be causing such large oscillations?

Nate

EE