CS4525 datasheet(58/98 Pages) CIRRUS

58 / 98 page

DS726PP1

CS4525

6.2.7

Automatic Power Stage Shut-Down

To protect itself from permanent damage, the CS4525 will automatically shut down its internal PWM pow-

er output stages when a thermal error, PWM power output over-current error, or VP under-voltage condi-

tion occurs. In the shut-down state, all digital functions of the device will operate as normal, however the

PWM power output pins become high-impedance.

The levels of the over-current error, thermal error, and VP under-voltage trigger points are listed in the

PWM Power Output Characteristics table on page 20. Shut-down will occur automatically whenever the

preset thresholds for thermal error or under-voltage are crossed.

When the over-current threshold is crossed, the CS4525 will attempt to automatically resume power out-

put operation after an over-current error is encountered and before placing its PWM power outputs in the

shut-down state. Upon the detection of an over-current condition, the CS4525 will place the PWM power

outputs in a high-impedance state, wait approximately 85 ms, and then re-engage the power outputs in

an attempt to resume normal operation. If another over-current condition is immediately detected, the

PWM power outputs will again be placed in a high-impedance state before retrying to resume normal op-

eration a second time. It will continue this sequence for a maximum of five attempts. After the fifth unsuc-

cessful attempt, the outputs will remain in the high-impedance shut-down state.

Once in the shut-down state, the RST signal must be toggled low and then high to resume normal device

operation.

6.3

PWM Modulators and Sample Rate Converters

The CS4525 includes three PWM modulators and three corresponding sample rate converters, each

clocked from the external crystal or system clock applied at power-up. All three modulator and sample rate

converter pairs are available in software mode (see Figure 14 on page 29), and two pairs are used in hard-

ware mode (see Figure 24 on page 56).

One of the characteristics of a PWM modulator is that the frequency content of the out-of-band noise gen-

erated is dependent on the PWM switching frequency. As the power stage external LC and snubber filter

component values are used to attenuate this out-of band energy, their component values are also based on

this switching frequency.

To easily accommodate input sample rates ranging from 32 kHz to 96 kHz without requiring the adjustment

of output filter component values, the CS4525 utilizes a sample rate converter (SRC) to keep the PWM

switching frequency fixed regardless of the input sample rate. The SRC operates by upsampling the variable

input sample rate to a fixed output switching rate, typically 384 kHz for most audio applications. Table 17

below shows the PWM output switching rate and quantization levels as a function of the supplied external

crystal or system clock.

Additionally, as the output of the SRC is clocked from a very stable crystal or oscillator, the SRC also allows

the PWM modulator output to be independent of the input serial audio clock jitter. This results in very low

jitter PWM output and higher dynamic range.

Supplied XTAL or

SYS_CLK Frequency

PWM Switch Rate

Quantization Levels

18.432 MHz

384 kHz

24.576 MHz

384 kHz

27.000 MHz

421.875 kHz

Table 17. PWM Output Switching Rates and Quantization Levels

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