Diodes AL8808 - Manual (Page 17)

AL8808
Document number: DS35648 Rev. 2 - 2
17 of 20
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January 2013
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AL8808
Application Information
(cont.)
EMI and Layout Considerations (cont.)
EMC Design
In addition to the layout instructions above, it may be necessary to take further measures to reduce electromagnetic interference (EMI) and meet
EMC requirements. This depends on the speed of the switching transitions. The fast switching edges include spectral harmonics spreading into
the UHF frequency range towards 500MHz. In this respect, AL8808 has been optimized to shape the switching current waveform to minimize EMI
while maintaining fast enough switching for high power efficiency. However, depending on the physical system design it may be necessary to add
additional filtering to reduce radiated and conducted emissions. The required circuit changes depend on a number of system design aspects
including the PCB size, the housing design and the length of external connecting wires.
Radiated Emission
Typically, the filtering required to control radiated emission consists of one or two additional capacitors placed close to the connecting points of the
wires. Very often the frequency range requiring most attenuation is in the region of 100MHz to 500MHz. In order to provide best attenuation in
this frequency range, use a capacitor of 1000pF to 2200pF with COG dielectric type, rated 50V or 100V. This capacitor provides very low ESR in
this frequency range. Place two such capacitors, one near the V
IN
wire connection and one near the output connection to L1. Again, to ensure
the best possible EMI filtering (greatest attenuation), place the capacitor and its copper trace such that the input or output current passes directly
through the capacitor mounting pad. This minimizes common impedance coupling due to the added parasitic inductance of the copper trace.
Conducted Emission
Conducted emission limits sometimes require filtering in the lower frequency range, from the switching frequency itself (Typically 200kHz) up to
about 30MHz. Usually the requirement only applies on the input side. The existing power supply may already include suitable measures. If
necessary add an input capacitor to reduce the ripple in this frequency range. Again the capacitors and their copper traces should be carefully
placed to avoid inductive common impedance coupling. Sometimes an additional series filter inductor may be added to achieve the desired
attenuation. An additional shunt capacitor to ground is connected resulting in a pi-filter configuration.