3 output filter power dissipation considerations, Cs35l00 – Cirrus Logic CS35L00 User Manual
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CS35L00
18
DS906PP1
allows the ferrite bead to sufficiently attenuate the problematic high-frequency emissions without compro-
mising audio performance.
5.3.3
Output Filter Power Dissipation Considerations
In systems without inductive series elements like inductors or ferrite beads, power losses in the output
filter are equal to the switching losses that occur in the system due to the cyclical charging and discharging
of capacitors connected to the amplifier outputs. In systems that require an inductive series element, con-
ducted losses also occur due to the series impedance added to the output path.
5.3.3.1
Conduction Losses for All Modes of Operation
For all modes of operation (SD, FSD, HD, and FHD), conduction losses are governed by the following
equation:
Where:
P
= Power dissipated in the series impedance.
I
= RMS AC output current
Z
= impedance of the series element at the frequency of the AC current
This equation neglects any series impedances presented by the PCB traces or speaker wires in the output
path.
5.3.3.2
Switching Losses in SD/FSD Mode
Switching losses in SD/FSD Mode are governed by the equation
Where:
P
= Power dissipated in the capacitor (neglecting parasites).
C
= Value of filtering capacitor
V
= Peak voltage developed across the capacitor
f
= Switching frequency of the outputs
These calculations are straightforward, as the peak voltage is simply the voltage level attached to VBATT,
the capacitor is the value of capacitor that has been added for filtering (neglecting parasitic board capac-
itances), and the frequency is 192 kHz for SD and 76 kHz for FSD, respectively.
5.3.3.3
Switching Losses in HD/FHD.
Many factors affect the switching losses when the device is operated in HD/FHD mode. These factors in-
clude the frequency of the content being amplified, the voltage level of VBATT, and the amplitude of the
output signal will factor into both the voltage presented across the capacitors and the frequency at which
the capacitors are charged or discharged.
P
I
2
Z
=
P
1
2
---CV
2
f
=