lm4985tmx National Semiconductor Corporation, lm4985tmx Datasheet - Page 25

• Manufacturer Part Number: lm4985tmx
• Description: Stereo 135mw Low Noise Headphone Amplifier With Selectable Capacitively Coupled Or Output Capacitor-less Ocl Output And Digitally Controlled I2c V
• Manufacturer: National Semiconductor Corporation

Application Information
coupling capacitor requires more charge to reach its quies-
cent DC voltage. This charge comes from the output via the
feedback Thus, by minimizing the capacitor size based on
necessary low frequency response, turn-on time can be
minimized. A small value of C
0.68µF), is recommended.
MAXIMIZING OCL MODE CHANNEL-to-CHANNEL
SEPARATION
The OCL mode AC ground return (CNT_GND pin) is shared
by both amplifiers. As such, any resistance between the
CNT_GND pin and the load will create a voltage divider with
respect to the load resistance. In a typical circuit, the amount
of CNT_GND resistance can be very small, but still signifi-
cant. It is significant because of the relatively low load im-
pedances for which the LM4985 was designed to drive: 16Ω
i
(in the range of 0.22µF to
(Continued)
25
to 32Ω. The ratio of this voltage divider will determine the
magnitude of any residual signal present at the CNT_GND
pin. It is this residual signal that leads to channel-to-channel
separation (crosstalk) degradation.
For example, for a 60dB channel-to-channel separation
while driving a 16Ω load, the resistance between the
LM4985’s CNT_GND pin and the load must be less than
16mΩ. This is achieved by ensuring that the trace that
connects the CNT_GND pin to the headphone jack sleeve
should be as short and massive as possible, given the
physical constraints of any specific printed circuit board lay-
out and design.
DEMONSTRATION BOARD AND PCB LAYOUT
Information concerning PCB layout considerations and dem-
onstration board use and performance is found in Application
Note AN-1452.
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