Pre-Production
WM8959
DISABLED OUTPUTS
Whenever an analogue output is disabled, it can be connected to VREF through a resistor; this
feature is enabled by setting the BUFIOEN bit – see “Pop Suppression Control”. This helps to
prevent pop noise when the output is re-enabled. The resistance between VREF and each output
can be controlled using register bit VROI. By default, a high resistance is used - 20kΩ for Headphone
outputs (LOUT, ROUT, OUT3 and OUT4) and 10kΩ for Line outputs (LON, LOP, RON and ROP). If
a low impedance is desired for disabled outputs, VROI can then be set to 1, decreasing the
resistance to about 500Ω in all cases.
Note that a disabled output may be used as a common ground connection for a capless headphone
output as described earlier.
REGISTER
ADDRESS
BIT
LABEL
VROI
DEFAULT
DESCRIPTION
R55 (37h)
0
0
VREF to Analogue Output Resistance
(Disabled Outputs)
Additional
Control
0 = 20kΩ (Headphone) or 10kΩ (Line
Out) from buffered VMID to output
1 = 500Ω from buffered VMID to output
Table 34 Disabled Outputs to VREF Resistance
THERMAL SHUTDOWN
The speaker and headphone outputs can drive very large currents. To protect the WM8959 from
overheating a thermal shutdown circuit is included. If the device temperature reaches approximately
150ºC and the thermal shutdown circuit is enabled (TSHUT_ENA = 1; TSHUT_OPDIS = 1) the
speaker and headphone amplifiers (LOUT, ROUT, SPKP, SPKN, OUT3 and OUT4) will be disabled.
TSHUT_ENA must be set to 1 to enable the temperature sensor when using the TSHUT_OPDIS
thermal shutdown function. The output of the temperature sensor can also be output to the GPIO
pins.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R2 (02h)
14
TSHUT_ENA
(rw)
1b
Thermal Sensor Enable
0 = Thermal sensor disabled
1 = Thermal sensor enabled
Thermal Shutdown Enable
13
TSHUT_OPDIS
(rw)
1b
(Requires thermal sensor to be
enabled)
0 = Thermal shutdown disabled
1 = Thermal shutdown enabled
Table 35 Thermal Shutdown
When the speaker driver is operating in class AB mode the internal power dissipation of the WM8959
is likely to be significantly higher than when operating in class D mode.
Note: To prevent potential pops and clicks THSUT_ENA and TSHUT_OPDIS need to be configured
while the speaker and headphone outputs are off, i.e. LOUT_ENA, ROUT_ENA, OUT3_ENA,
OUT4_ENA and SPK_ENA are 0 (see also Table 70).
PP, May 2008, Rev 3.1
71
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
