WM8978 Wolfson Microelectronics Ltd., WM8978 Datasheet - Page 54

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WM8978

Manufacturer Part Number
WM8978
Description
The WM8978 is a low power, high quality stereo codec designed for portable applications such as Digital still camera or Digital Camcorde
Manufacturer
Wolfson Microelectronics Ltd.
Datasheet

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WM8978
Headphone Output using DC Blocking Capacitors:
Figure 25 Recommended Headphone Output Configurations
w
SPEAKER OUTPUTS (LOUT2 AND ROUT2)
When DC blocking capacitors are used, then their capacitance and the load resistance together
determine the lower cut-off frequency, f
response. Smaller capacitance values will diminish the bass response. Assuming a 16
C1, C2 = 220 F:
f
In the DC coupled configuration, the headphone “ground” is connected to the VMID pin. The
OUT3/4 pins can be configured as a DC output driver by setting the OUT3MUTE and OUT4MUTE
register bit. The DC voltage on VMID in this configuration is equal to the DC offset on the LOUT1
and ROUT1 pins therefore no DC blocking capacitors are required. This saves space and material
cost in portable applications.
Note that OUT3 and OUT4 have an optional output boost of 1.5x. When these are configured in this
output boost mode (OUT3BOOST/OUT4BOOST=1) then the VMID value of these outputs will be
equal to 1.5xAVDD/2 and will not match the VMID of the headphone drivers. Do not use the DC
coupled output mode in this configuration.
It is recommended to connect the DC coupled outputs only to headphones, and not to the line input
of another device. Although the built-in short circuit protection will prevent any damage to the
headphone outputs, such a connection may be noisy, and may not function properly if the other
device is grounded.
The outputs LOUT2 and ROUT2 are designed to drive an 8Ω BTL speaker but can optionally drive
two headphone loads of 16 /32
respectively). Each output has an individual volume control PGA, an output boost/level shift bit, a
mute and an enable as shown in Figure 26.
mixer outputs respectively.
The ROUT2 signal path also has an optional invert. The amplifier used for this invert can be used to
mix in the AUXR signal with an adjustable gain range of -15dB -> +6dB. This allows a ‘beep’ signal
to be applied only to the speaker output without affecting the HP or line outputs.
c
= 1 / 2 R
L
C
1
= 1 / (2 x 16
x 220 F) = 45 Hz
or a line output (see Headphone Output and Line Output sections,
c
DC Coupled Headphone Output:
. Increasing the capacitance lowers f
LOUT2 and ROUT2 output the left and right channel
PTD Rev 2.6 November 2005
Preliminary Technical Data
c
, improving the bass
load and
54