LM4867MT National Semiconductor, LM4867MT Datasheet - Page 14

• Manufacturer Part Number: LM4867MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

www.national.com
Application Information
Truth Table for Logic Inputs
HP-IN FUNCTION
An internal pull−up circuit is connected to the HP−IN (pin 20)
headphone amplifier control pin. When this pin is left uncon-
nected, V
and switches Amp2A’s input signal from an audio signal to
the V
bridge-connected loads. Quiescent current consumption is
reduced when the IC is in this single−ended mode.
Figure 7 shows the implementation of the LM4867’s head-
phone control function. An internal comparator with a nomi-
nal 400mV offset monitors the signal present at the −OUTB
output. It compares this signal against the signal applied to
the HP−IN pin. When these signals are equal, as is the case
when a BTL is connected to the amplifier, the comparator
forces the LM4867 to maintain bridged−amplifier operation.
When the HP−IN pin is externally floated, such as when
headphones are connected to the jack shown in Figure 7 ,
and internal pull−up forces V
HP−IN inputs. This changes the comparator’s output state
and enables the headphone function: it turns off Amp2B,
switches Amp2A’s input signal from an audio signal to the
V
bridge-connected loads. Amp1A and Amp1B drive the head-
phones.
Figure 7 also shows the suggested headphone jack electri-
cal connections. The jack is designed to mate with a
three−wire plug. The plug’s tip and ring should each carry
one of the two stereo output signals, whereas the sleeve
provides the return to Amp2A. A headphone jack with one
control pin contact is sufficient to drive the HP−IN pin when
connecting headphones.
A switch can replace the headphone jack contact pin. When
a switch shorts the HP−IN pin to V
speakers are muted and Amp1A and Amp2A drive a pair of
DD
/2 voltage present on pin 14, and mutes the
DD
/2 voltage present on pin 14. The result is muted
DD
is applied to the HP−IN. This turns off Amp2B
SHUTDOWN
Logic High
Logic Low
Logic Low
Logic Low
Logic Low
PIN
DD
on the internal comparator’s
DD
= −OUTB signal
= −OUTB signal
, bridge−connected
−OUTB signal
−OUTB signal
(Continued)
HP-IN
PIN
X
MUX CHANNEL
INPUT SELECT
14
Logic High
Logic High
Logic Low
Logic Low
headphones. When a switch shorts the HP−IN pin to GND,
the LM4867 operates in bridge mode. If headphone drive is
not needed, short the HP−IN pin to the −OUTB pin.
Single-Ended Output Power Performance and
Measurement Considerations
The LM4867 delivers clean, low distortion SE output power
into loads that are greater than 10 . As an example, output
power for 16
Performance Characteristic curves. For loads less than
10 , the LM4876 can typically supply 180mW of low distor-
tion power. However, when higher dissipation is desired in
loads less than 10 , a dramatic increase in THD+N may
occur. This is normal operation and does not indicate that
proper functionality has ceased. When a jump from moder-
ate to excessively high distortion is seen, simply reducing
the output voltage swing will restore the clean, low distortion
SE operation.
The dramatic jump in distortion for loads less than 10
occurs when current limiting circuitry activates. During SE
operation, AMP2A (refer to Figure 4 ) drives the headphone
sleeve. An on-board circuit monitors this amplifier’s output
current. The sudden increase in THD+N is caused by the
current limit circuitry forcing AMP2A into a high−impedance
output mode. When this occurs, the output waveform has
discontinuities that produce large amounts of distortion. It
has been observed that as the output power is steadily
increased, the distortion may jump from 5% to greater than
35%. Indeed, 10% THD+N may not actually be achievable.
Using the Single−Ended Output for Line Level
Applications
Some samples of the LM4867 may exhibit small amplitude,
high frequency oscillation when the SE output is connected
to a line-level input. This oscillation can be eliminated by
connecting a 5%, 300 resistor between Amp2A’s output pin
and each amplifier, AMP1A and AMP1B, output.
X
OPERATIONAL MODE (MUX
and 32
Single-ended amplifiers (1)
Single-ended amplifiers (2)
Micro−power shutdown
Bridged amplifiers (1)
Bridged amplifiers (2)
INPUTCHANNEL #)
loads are shown in the Typical