CS3511

16

DS845F1

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.

Load

L1, L2

C1

8

Ω

22 µH

0.47 µF

6

Ω

15 µH

0.47 µF

Table 2. Low-Pass Filter Components

OUTx+

OUTx-

680 pF

5.6

Ω

C1

L1

L2

5.6

Ω

680 pF

Figure 4. Output Filter