參數(shù)資料
型號(hào)： LM4892M/NOPB
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 音頻／視頻放大
英文描述： 1 W, 1 CHANNEL, AUDIO AMPLIFIER, PDSO8
封裝： SOIC-8
文件頁(yè)數(shù)： 3/22頁(yè)
文件大?。?/td> 1037K
代理商： LM4892M/NOPB
Application Information (Continued)
requirements, click and pop performance (as explained in
the section, Proper Selection of External Components),
system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4892 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the
amplifier off when a logic low is placed on the shutdown pin.
By switching the shutdown pin to ground, the LM4892 supply
current draw will be minimized in idle mode. While the device
will be disabled with shutdown pin voltages less than
0.5V
DC, the idle current may be greater than the typical
value of 0.1A. (Idle current is measured with the shutdown
pin grounded).
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry to provide a
quick, smooth transition into shutdown. Another solution is to
use a single-pole, single-throw switch in conjunction with an
external pull-up resistor. When the switch is closed, the
shutdown pin is connected to ground and disables the am-
plifier. If the switch is open, then the external pull-up resistor
will enable the LM4892. This scheme guarantees that the
shutdown pin will not float thus preventing unwanted state
changes.
Table 1. Logic Level Truth Table for Shutdown and HP
Sense Operation
Shutdown
HP Sense
Pin
Operational Mode
Logic High
Logic Low
Bridged Amplifier
Logic High
Single-Ended Amplifier
Logic Low
Micro-Power Shutdown
Logic Low
Logic High
Micro-Power Shutdown
HP SENSE FUNCTION
Applying a voltage between 4V and V
CC to the LM4892’s
HP-Sense headphone control pin turns off Amp2 and mutes
a bridged-connected load. Quiescent current consumption is
reduced when the IC is in the single-ended mode.
Figure 2 shows the implementation of the LM4892’s head-
phone control function. With no headphones connected to
the headphone jack, the R
4-R6 voltage divider sets the volt-
age applied to the HP-Sense pin (pin3) at approximately
50mV. This 50mV enables the LM4892 and places it in
bridged mode operation.
While the LM4892 operates in bridged mode, the DC poten-
tial across the load is essentially 0V. Since the HP-Sense
threshold is set at 4V, even in an ideal situation, the output
swing can not cause a false single-ended trigger. Connect-
ing headphones to the headphone jack disconnects the
headphone jack contact pin from V
01 and allows R4 to pull
the HP Sense pin up to V
CC. This enables the headphone
function, turns off Amp2, and mutes the bridged speaker.
The amplifier then drives the headphone whose impedance
is in parallel with R
6. Resistor R6 has negligible effect on
output drive capability since the typical impedance of head-
phones is 32
. The output coupling capacitor blocks the
amplifier’s half supply DC voltage, protecting the head-
phones.
A microprocessor or a switch can replace the headphone
jack contact pin. When a microprocessor or switch applies a
voltage greater than 4V to the HP Sense pin, a bridged-
connected speaker is muted and Amp1 drives the head-
phones.
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 LM4892 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4892 is unity-gain stable which gives the designer
maximum system flexibility. The LM4892 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.
Input signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the
section, Audio Power Amplifier Design, for a more com-
plete explanation of proper gain selection.
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
width is dictated by the choice of external components
shown in Figure 1. The input coupling capacitor, C
i, forms a
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed
frequency response for a few distinct reasons.
Selection Of Input Capacitor Size
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenu-
ation. But in many cases the speakers used in portable
systems, whether internal or external, have little ability to
reproduce signals below 100Hz to 150Hz. Thus, using a
large input capacitor may not increase actual system perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
C
i. A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 V
DD). This
charge comes from the output via the feedback and is apt to
20012774
FIGURE 2. Headphone Circuit (Pin #’s apply toM&MM
packages)
LM4892
www.national.com
11
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