LM4854 National Semiconductor, LM4854 Datasheet - Page 18

• Manufacturer Part Number: LM4854
• Description: 1.9W Monaural/ 85mW Stereo Headphone Audio Amplifier
• Manufacturer: National Semiconductor

www.national.com
Typical Performance Characteristics
Application Information
ELIMINATING OUTPUT COUPLING CAPACITORS
Typical single-supply audio amplifiers that can switch be-
tween driving bridge-tied-load (BTL) speakers and single-
ended (SE) headphones use a coupling capacitor on each
SE output. This capacitor blocks the half-supply voltage to
which the output amplifiers are typically biased and couples
the audio signal to the headphones. The signal returns to
circuit ground through the headphone jack’s sleeve.
The LM4854 eliminates these coupling capacitors. When the
LM4854 is configured to drive SE loads, AMP2 is internally
configured to apply V
sleeve. This voltage equals the quiescent voltage present on
the Amp1 and Amp3 outputs that drive the headphones.
A = Left channel driven, right channel measured
B = Right channel driven, left channel measured
A = Left channel driven, right channel measured
B = Right channel driven, left channel measured
Channel-to-Channel Crosstalk
Channel-to-Channel Crosstalk
V
V
DD
DD
= 5V, R
= 3V, R
DD
vs Frequency
vs Frequency
/2 to a stereo headphone jack’s
L
L
= 16Ω (SE)
= 16Ω (SE)
200382D4
200382D6
18
(Continued)
Headphones driven by the LM4854 operate in a manner very
similar to a BTL load. The same DC voltage is applied to
each input terminal on a headphone speaker. This results in
no net DC current flow through the speaker. AC current flows
through a headphone speaker as an audio signal’s output
amplitude increases on one of the speaker’s terminal.
When operating as a headphone amplifier, the headphone
jack sleeve is not connected to circuit ground, but to V
Using the headphone output jack as a line-level output will
place the LM4854’s one-half supply voltage on a plug’s
sleeve connection. Driving a portable notebook computer or
audio-visual display equipment is possible. This presents no
difficulty when the external equipment uses capacitively
coupled inputs. For the very small minority of equipment that
is DC-coupled, the LM4854 monitors the current supplied by
A = Left channel driven, right channel measured
B = Right channel driven, left channel measured
A = Left channel driven, right channel measured
B = Right channel driven, left channel measured
Channel-to-Channel Crosstalk
Channel-to-Channel Crosstalk
V
V
DD
DD
= 5V, R
= 3V, R
vs Frequency
vs Frequency
L
L
= 32Ω (SE)
= 32Ω (SE)
200382D5
200382D7
DD
/2.