CS44800 datasheet(32/80 Pages) CIRRUS

32 / 80 page

DS632F1

CS44800

4.5

FsOut Clock Domain Modules

4.5.1

Sample Rate Converter

One of the characteristics of a PWM amplifier is that the frequency content of out-of-band noise generated

by the modulator is dependent on the PWM switching frequency. The power stage external LC and snub-

ber filter component values are based on this switching frequency. To accommodate input sample rates

ranging from 32 kHz to 192 kHz the CS44800 utilizes a Sample Rate Converter (SRC) and several clock-

ing modes that keep the PWM switching frequency fixed.

The SRC supports a range of sample rate conversion to upsample rates from 32 kHz to 192 kHz to a fixed

FsOut sample rate. This is typically 384 kHz for most audio applications. The SRC also allows the PWM

modulator output to be independent of the input clock jitter since the output of the SRC is clocked from a

very stable crystal or oscillator. This results in very low jitter PWM output and higher dynamic range.

4.5.2

Load Compensation Filter

To accommodate varying speaker impedances, the CS44800 incorporates a 2-pole load compensation

filter to adjust the effective frequency response of the on-card L/C de-modulation filter. The frequency re-

sponse of the 2-pole inductor/capacitor filter used on the board to filter out the high-frequency PWM

switching clock is highly dependant on the resistive load (speaker) attached.

If the L/C filter implemented was designed for a low impedance load (4

Ω speaker), but an 8 Ω speaker

was attached, the frequency response would have a large peaking near the resonant frequency of the

L/C. The peaking usually starts at around 15 kHz, with about a +4 dB of gain at around 20 kHz. This phe-

nomenon will cause the system to not meet the frequency response requirements as specified by Dolby

Labs.

By using the programmable 2-pole load compensation filter, the overall frequency response of the system

can be modified to cut the amount of peaking. The 2 poles of the filter are independently configurable and

are concatenated to form the overall filter response. The first filter is defined as a coarse setting. This filter

should be programmed to provide most of the attenuation of the peaking. The second filter, defined as the

fine adjust, is used to achieve incremental improvements to the overall frequency response. Table 3

shows example register settings based on an output filter that has been designed for a 4

Ω load imped-

ance. See “Channel Compensation Filter - Coarse Adjust (CHXX_CORS[5:0])” on page 62 and “Channel

Compensation Filter - Fine Adjust (CHXX_FINE[5:0])” on page 63.

4.5.3

Digital Volume and Mute Control

The CS44800 provides two levels of volume control. A Master Volume Control Register is used to set the

volume level across all PWM channels. The register value, which selects a volume range of +24 dB to -

127 dB in 0.25 dB steps, is used to control the overall volume setting of all the amplifier channels. Volume

control changes are programmable to ramp in increments of 0.125 dB at a variable rate controlled by the

SZC[1:0] bits in “Volume Control Configuration (address 06h)” on page 55.

Each PWM channel’s output level is controlled via a Channel Volume Control register operating over the

range of +24 dB to -127 dB attenuation with 0.25 dB resolution. See “Channel XX Volume Control - Inte-

Load Impedance

Coarse Filter Setting

Fine Filter Setting

Ω

-1.2 dB

0 dB

Ω

-1.8 dB

0 dB

Ω

-3.4 dB

0 dB

Table 3. Load Compensation Example Settings

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