LM4985TM/NOPB National Semiconductor, LM4985TM/NOPB Datasheet - Page 25

• Manufacturer Part Number: LM4985TM/NOPB
• Description: IC AMP AUDIO PWR .135W AB 12USMD
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4985TM/NOPB

• Output Type: Headphones, 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 135mW x 2 @ 16 Ohm
• Voltage - Supply: 2.3 V ~ 5.5 V
• Features: I²C, Shutdown, Thermal Protection, Volume Control
• Mounting Type: Surface Mount
• Package / Case: 12-MicroSMD
• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 2-Channel Stereo
• Output Power (typ): 135x2@16OhmW
• Audio Amplifier Function: Headphone
• Single Supply Voltage (typ): 3/5V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 77dB
• Single Supply Voltage (min): 2.3V
• Single Supply Voltage (max): 5.5V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 12
• For Use With: LM4985TMEVAL - BOARD EVALUATION LM4985TM
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM4985TMTR

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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