CS3511_0912 Datasheet, PDF (16/26 Pages) Cirrus Logic – Stereo 10 W High-efficiency Class-D Audio Power Amplifier

CS3511
continuous heavy load conditions. There should be vias for connecting the exposed pad to the copper area
on the printed circuit board. The pad must be electrically connected to PGND. See Section 5.2 for more in-
formation on the thermal pad and Section 9.1 for more information on thermal dissipation for the CS3511.
4.10 Performance Measurements of the CS3511
The CS3511 operates by generating a high-frequency switching signal based on the audio input. This signal
is sent through a low-pass filter (external to the CS3511 amplifier) that recovers an amplified version of the
audio input. The frequency of the switching pattern is spread spectrum and typically varies between 100 kHz
and 1.0 MHz, which is well above the 10 Hz – 20 kHz audio band. The pattern itself does not alter or distort
the audio input signal, but it does introduce some inaudible components outside of the audio band.
The measurements of certain performance parameters, particularly noise-related specifications such as
THD+N, are significantly affected by the design of the low-pass filter used on the output as well as the band-
width setting of the measurement instrument used. Unless the filter has a very sharp roll-off just beyond the
audio band or the bandwidth of the measurement instrument is limited, some of the inaudible components
introduced by the CS3511 amplifier’s switching pattern will degrade the measurement result.
One feature of the CS3511 is that it does not require large multi-pole filters to achieve excellent performance
in listening tests, usually a more critical factor than performance measurements. The CRD3511 Evaluation
Board uses the filter described in Section 4.11, which has a simple two-pole output filter and excellent per-
formance in listening tests. Measurements in this data sheet were taken using this same circuit with a limited
bandwidth setting in the measurement instrument.
4.11 Full-Bridge Output Filter
Figure 4 shows the output filter for a full-bridge configuration. The transient-voltage suppression circuit
(snubber circuit) is comprised of a resistor (5.6 Ω) and capacitor (680 pF) and should be placed as close
as possible to the corresponding PWM output pins to greatly reduce radiated EMI. The inductors, L1 and
L2, and capacitor, C1, comprise the low-pass filter. Along with the nominal load impedance of the speaker,
these values set the cutoff frequency of the filter. Table 2 shows the component values based on nominal
speaker (load) impedance for a corner frequency (-3 dB point) of approximately 35 kHz.
L1
OUTx+
5.6 Ω
680 pF
C1
L2
OUTx-
5.6 Ω
680 pF
Figure 4. Output Filter
Load
8Ω
6Ω
L1, L2
22 µH
15 µH
C1
0.47 µF
0.47 µF
Table 2. Low-Pass Filter Components
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