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12.0 Status & Control Registers (Continued)
12.17 MIC_2 CONFIGURATION REGISTER
This register is used to control the microphone configuration.
TABLE 17. MIC_2 (0x0Ch)
Bits
Field
Description
0
OCL_
VCM_
VOLTAGE
Selects the voltage used as virtual ground (HP_VMID pin) in OCL mode. This will depend on the
available supply and the power output requirements of the headphone amplifiers.
OCL_VCM_VOLTAGE
Voltage
0
1.2V
1
1.5V
2:1
MIC_
BIAS_
VOLTAGE
Selects the voltage as a reference to the internal and external microphones. Only one bias pin is
driven at once depending on the INT_EXT bit setting found in the MIC_1 (0x0Bh) register.
MIC_BIAS_VOLTAGE should be set to ’11’ only if A_V
DD > 3.4V. In OCL mode,
MIC_BIAS_VOLTAGE = ’00’ (EXT_BIAS = 2.0V) should not be used to generate the EXT_BIAS
supply for a cellular headset external microphone. Please refer to Table 18 for more detail.
MIC_BIAS_VOLTAGE
EXT_BIAS
INT_BIAS
00
2
2.0V
01
2
2.5V
10
2
2.8V
11
2
3.3V
3
BUTTON_TYPE
If set the LM4935 assumes that the button (if used) in the headset is in series (series push button)
with the microphone, opening the circuit when pressed. The default is for the button to be in parallel
(parallel push button), shorting out the microphone when pressed.
5:4
BUTTON_
DEBOUNCE_
TIME
Sets the time used for debouncing the pushing of the button on a headset with a parallel push
button.
BUTTON_DEBOUNCE_TIME
Time (ms)
00
2
0
01
2
8
10
2
16
11
2
32
In OCL mode there is a trade-off between the external microphone supply voltage (EXT_MIC_BIAS - OCL_VCM_ VOLTAGE) and
the maximum output power possible from the headphones. A lower OCL_VCM_VOLTAGE gives a higher microphone supply
voltage but a lower maximum output power from the headphone amplifiers due to the lower OCL_VCM_VOLTAGE - A_V
SS.
TABLE 18. External MIC Supply Voltages in OCL Mode
Available
A_V
DD
Recommended
EXT_MIC_BIAS
Supply to Microphone
OCL_VCM_VOLT = 1.5V
OCL_VCM_VOLT = 1.2V
> 3.4V
3.3V
1.8V
2.1V
2.9V to 3.4V
2.8V
1.3V
1.6V
2.8V to 2.9V
2.5V
1.0V
1.3V
2.7V to 2.8V
2.5V
-
1.3V
LM4935
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