20205210
Horizontal Resolution: 5 µs/div.
Trace 1: The AUX input current ripples (AC coupled). 10 mA/div.
Trace 2: The FFT of Trace 1 showing the peak value of the harmonics. Hor-
izontal 500 kHz/div. Vertical 2.5 mA/div.
FIGURE 12. AUX Input Current Ripple Under Full Load
Figure 13 shows the output ripple voltage. The FFT of the
output ripple voltage indicates that the ripple harmonics are
below 15 mV pk-pk.
20205211
Horizontal Resolution: 50 µs/div.
Trace 1: The 3.3V output voltage ripples (AC coupled). 0.2V/div
Trace 2: The FFT of Trace 1 showing the peak value of the harmonics. Hor-
izontal 500 kHz/div. Vertical 5 mV/div
FIGURE 13. Output Ripple Voltage under Full Load
A Note on the Use of Common-Mode
Choke
A location is reserved on the evaluation board for an optional
input common mode filter. For some special applications that
have very strict EMI requirements, the common mode filter
consisting of the choke U6 and two Y capacitors CE12 and
CE13 can be used. On the original evaluation board, these
three components are not populated, and U6’s pads are
shorted with bus wires. When a common mode filter is re-
quired, a Coilcraft D1882-AL or equivalent can be used for
U6, along with a Syfer 1808JA250102MCTPY2 or equivalent
for CE12 and CE13.
www.national.com
12
AN-1521
