WM8994ECSSLASHR datasheet(351/363 Pages) CIRRUS

351 / 363 page

WM8994

Rev 4.6

351

EARPIECE DRIVER OUTPUT PATH

The earpiece driver on HPOUT2P and HPOUT2N is designed as a 32ohm BTL speaker driver. The

outputs are referenced to the internal DC reference VMID, but direct connection to the speaker is

possible because of the BTL configuration. There is no requirement for DC blocking capacitors.

LINE OUTPUT PATHS

The WM8994 provides four line outputs (LINEOUT1P, LINEOUT1N, LINEOUT2P and LINEOUT2N).

Each of these outputs is referenced to the internal DC reference, VMID. In any case where a line

output is used in a single-ended configuration (i.e. referenced to AGND), a DC blocking capacitor will

be required in order to remove the DC bias. In the case where a pair of line outputs is configured as a

BTL differential pair, then the DC blocking capacitor should be omitted.

The choice of capacitor is determined from the filter that is formed between the capacitor and the load

impedance

– see Figure 88.

Fc = high pass 3dB cut-off frequency

Output

Fc =

p RC

Figure 88 Line Output Path Components

LOAD IMPEDANCE

MINIMUM CAPACITANCE

FOR 20HZ PASS BAND

10k



0.8

F

47k



0.17

F

Table 147 Line Output Frequency Cut-Off

Using the figures in Table 147, it follows that that a 1

F capacitance would be a suitable choice for a

line load. Tantalum electrolytic capacitors are again particularly suitable but ceramic equivalents are a

cost effective alternative. Care must be taken to ensure the desired capacitance is maintained at the

appropriate operating voltage.

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