WM8991 Datasheet, PDF (81/174 Pages) Wolfson Microelectronics plc – Mobile Multimedia CODEC with Dual-Mode Class AB/D Speaker Driver

Pre-Production
WM8991
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
R55 (37h)
Additional
Control
BIT
LABEL
0
VROI
DEFAULT
0
Table 42 Disabled Outputs to VREF Resistance
DESCRIPTION
VREF to Analogue Output Resistance
(Disabled Outputs)
0 = 20kΩ (Headphone) or 10kΩ (Line
Out) from buffered VMID to output
1 = 500Ω from buffered VMID to output
THERMAL SHUTDOWN
The speaker and headphone outputs can drive very large currents. To protect the WM8991 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
R2 (02h)
BIT
LABEL
14 TSHUT_ENA
(rw)
13 TSHUT_OPDIS
(rw)
Table 43 Thermal Shutdown
DEFAULT
DESCRIPTION
1b
Thermal Sensor Enable
0 = Thermal sensor disabled
1 = Thermal sensor enabled
1b
Thermal Shutdown Enable
(Requires thermal sensor to be
enabled)
0 = Thermal shutdown disabled
1 = Thermal shutdown enabled
When the speaker driver is operating in class AB mode the internal power dissipation of the WM8991
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 79).
w
PP, May 2008, Rev 3.1
81