LM4862M National Semiconductor, LM4862M Datasheet - Page 8

• Manufacturer Part Number: LM4862M
• Description: IC AMP AUDIO PWR .825W AB 8SOIC
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4862M

• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 825mW x 1 @ 8 Ohm
• Voltage - Supply: 2.7 V ~ 5.5 V
• Features: Shutdown, Thermal Protection
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LM4862M

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Application Information
AUTOMATIC SWITCHING CIRCUIT
As shown in Figure 2, the LM4862 and the LM4880 can be
set up to automatically switch on and off depending on
whether headphones are plugged in. The LM4880 is used to
drive a stereo single ended load, while the LM4862 drives a
bridged internal speaker.
The Automatic Switching Circuit is based upon a single
control pin common in many headphone jacks which forms a
normally closed switch with one of the output pins. The
output of this circuit (the voltage on pin 5 of the LM4880) has
two states based on the position of the switch. When the
switch inside the headphone jack is open, the LM4880 is
enabled and the LM4862 is disabled since the NMOS in-
verter is on. If a headphone jack is not present, it is assumed
that the internal speakers should be on and the external
speakers should be off. Thus the voltage on the LM4862
shutdown pin is low and the voltage on the LM4880 shut-
down pin is high.
The operation of this circuit is rather simple. With the switch
closed, R
gate voltage of less than 50 mV. The gate voltage keeps the
NMOS inverter off and R
LM4880 to the supply voltage. This shuts down the LM4880
and places the LM4862 in its normal mode of operation.
When the switch is open, the opposite condition is produced.
Resistor R
the inverter and produces a logic low signal on the shutdown
pin of the LM4880. This state enables the LM4880 and
places the LM4862 in shutdown mode.
Only one channel of this circuit is shown in Figure 2 to keep
the drawing simple but a typical application would be a
LM4880 driving a stereo headphone jack and two LM4862’s
driving a pair of internal speakers. If a single internal speaker
is required, one LM4862 can be used as a summer to mix
the left and right inputs into a mono channel.
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 LM4862 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4862 is unity-gain stable which gives a designer
maximum system flexibility. The LM4862 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
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.
P
P
and R
pulls the gate of the NMOS high which turns on
O
form a resistor divider which produces a
SD
pulls the shutdown pin of the
(Continued)
i
, forms a
8
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 100–150 Hz. Thus using a large
input capacitor may not increase system performance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
C
reach its quiescent DC voltage (nominally
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the
capacitor size based on necessary low frequency response,
turn-on pops can be minimized.
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
turn-on pops since it determines how fast the LM4862 turns
on. The slower the LM4862’s outputs ramp to their quiescent
DC voltage (nominally
Choosing C
(in the range of 0.1 µF to 0.39 µF), should produce a virtually
clickless and popless shutdown function. While the device
will function properly, (no oscillations or motorboating), with
C
to turn-on clicks and pops. Thus, a value of C
1.0 µF or larger is recommended in all but the most cost
sensitive designs.
AUDIO POWER AMPLIFIER DESIGN
Design a 500 mW/8Ω Audio Amplifier
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the Typical Per-
formance Characteristics section, the supply rail can be
easily found. A second way to determine the minimum sup-
ply rail is to calculate the required V
and add the dropout voltage. Using this method, the mini-
mum supply voltage would be (V
is extrapolated from the Dropout Voltage vs Supply Voltage
curve in the Typical Performance Characteristics section.
Using the Output Power vs Supply Voltage graph for an 8Ω
load, the minimum supply rail is 4.3V. But since 5V is a
standard supply voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4862 to reproduce peaks in excess of 500 mW
without clipping the signal. At this time, the designer must
make sure that the power supply choice along with the
output impedance does not violate the conditions explained
in the Power Dissipation section.
Given:
i
B
. A larger input coupling capacitor requires more charge to
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
equal to 0.1 µF, the device will be much more susceptible
B
B
, is the most critical component to minimize
equal to 1.0 µF along with a small value of C
1
⁄
2
V
DD
), the smaller the turn-on pop.
opeak
100 Hz–20 kHz
opeak
+ (2*V
using equation 3
OD
1
)), where V
⁄
2
500 mWrms
V
±
B
DD
equal to
0.25 dB
1 Vrms
). This
20 kΩ
8Ω
(3)
OD
i