LM4873MT National Semiconductor, LM4873MT Datasheet - Page 12

• Manufacturer Part Number: LM4873MT
• Description: Dual 2.1W Audio Amplifier Plus Stereo Headphone Function
• Manufacturer: National Semiconductor

www.national.com
Application Information
sistor will disable the LM4873. This scheme guarantees that
the shutdown pin will not float, thus preventing unwanted
state changes.
HP-IN FUNCTION
The LM4873 possesses a headphone control pin that turns
off the amplifiers which drive +OutA and +OutB so that
single-ended operation can occur and a bridged connected
load is muted. Quiescent current consumption is reduced
when the IC is in this single-ended mode.
Figure 3 shows the implementation of the LM4873’s head-
phone control function using a single-supply headphone am-
plifier. The voltage divider of R1 and R2 sets the voltage at
the HP-IN pin (pin 16) to be approximately 50 mV when there
are no headphones plugged into the system. This logic-low
voltage at the HP-IN pin enables the LM4873 and places it in
bridged mode operation. Resistor R4 limits the amount of
current flowing out of the HP-IN pin when the voltage at that
pin goes below ground resulting from the music coming from
the headphone amplifier. The output coupling capacitors pro-
tect the headphones by blocking the amplifier’s half supply
DC voltage.
When there are no headphones plugged into the system and
the IC is in bridged mode configuration, both loads are es-
sentially at a 0V DC potential. Since the HP-IN threshold is
set at 4V, even in an ideal situation, the output swing cannot
cause a false single-ended trigger.
When a set of headphones are plugged into the system, the
contact pin of the headphone jack is disconnected from the
signal pin, interrupting the voltage divider set up by resistors
The LM4873 is unity-gain stable, giving the designer maxi-
mum system performance. The LM4873 should be used in
low gain configurations to minimize THD+N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power. In-
put signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the sec-
tion, Audio Power Amplifier Design, for a more complete
explanation of proper gain selection.
(Continued)
FIGURE 3. Headphone Circuit
12
R1 and R2. Resistor R1 then pulls up the HP-IN pin, en-
abling the headphone function. This disables the second
side of the amplifier thus muting the bridged speakers. The
amplifier then drives the headphones, whose impedance is
in parallel with resistors R2 and R3. Resistors R2 and R3
have negligible effect on output drive capability since the
typical impedance of headphones are 32 . Also shown in
Figure 3 are the electrical connections for the headphone
jack and plug. A 3-wire plug consists of a Tip, Ring and
Sleave, where the Tip and Ring are signal carrying conduc-
tors and the Sleave is the common ground return. One con-
trol pin contact for each headphone jack is sufficient to indi-
cate to control inputs that the user has inserted a plug into a
jack and that another mode of operation is desired.
The LM4873 can be used to drive both a pair of bridged 8
speakers and a pair of 32
HP-IN pin. In this case the HP-IN would not be connected to
the headphone jack but to a microprocessor or a switch. By
enabling the HP-IN pin, the 8
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4873 is tolerant to a
variety of external component combinations, consideration
to component values must be used to maximize overall sys-
tem quality.
Besides gain, one of the major considerations is the
closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components
shown in Figure 1 . The input coupling capacitor, C
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed fre-
quency response for a few distinct reasons.
DS100993-24
headphones without using the
speakers can be muted.
i
, forms a