TYPICAL APPLICATIONS
EMC AND EMI RECOMMENDATIONS
RC Out
Filter C2
RC In
Filter
PI
Filter
Duty Cycle
Increase
I(t) on VBAT
t
Figure 27. VPWR Current
C3
HOW TO REDUCE ELECTROMAGNETIC EMISSION
By adjusting the slew rate of the device during turn ON and
turn OFF with SR capacitor, the electromagnetic emissions
can be reduced.
C1
SR
Conductive emission tests were performed (taking care of
the board filtering and routing that have a big impact on EMC
performances).
An optimized solution was found by adding the following
external components to the initial board:
• PI filter on the VPWR: 2 x 3 mF and 3.5 uH
• RC IN filter between VPWR and GND: a 2.0 Ω resistor in
series with a 100 nF capacitor
• RC Out filter between OUT and GND: a 4.7 Ω resistor
in series with a 100 nF capacitor
• Capacitor C1 of 10 nF between VPWR and GND
• Capacitor C2 of 10 nF between OUT and GND
• Capacitor C3 of 10 nF between OUT and VPWR
• Capacitor SR of 3.3 nF
Figure 29. Enhanced Board
The chart in Figure 30 shows the spectrum of the
enhanced board and the initial board. The improvement is
appreciatively 15 dB to 20 dB in the all frequency range. The
enhanced board is now in accordance with the Class 3 limits
of the CISPR25 standard for conducted emission.
C3 = 10 nF
RC In Filter
RC Out Filter
PI filter
3.5 µH
OUT
VBAT
33981
33891
3000 µF
SR
3.3 nF
GND
Figure 28. 33981 with Filter
The EMC enhanced board with adapted value filter is
represented in Figure 29, Enhanced Board, on page 25.
Figure 30. Conducted Emission Spectrum for 33981
RADIATED MEASUREMENTS
This test was performed in order to evaluate the
characteristic of the device relating to radiated emission.
Measurements have been done in accordance with the
33981
Analog Integrated Circuit Device Data
Freescale Semiconductor
25
FREESCALE [ Freescale ]
