LM4877 National Semiconductor, LM4877 Datasheet - Page 9

• Manufacturer Part Number: LM4877
• Description: 1 Watt Audio Power Amplifier in micro SMD package with Shutdown Logic Low
• Manufacturer: National Semiconductor

Application Information
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 fre-
quency 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 attenua-
tion. But in many cases the speakers used in portable sys-
tems, whether internal or external, have little ability to repro-
duce signals below 100 Hz to 150 Hz. 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
reach its quiescent DC voltage (nominally 1/2 V
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the ca-
pacitor 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 LM4877 turns
on. The slower the LM4877’s outputs ramp to their quiescent
DC voltage (nominally 1/2 V
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 is recommended in all but the most cost sensitive de-
signs.
LOW VOLTAGE APPLICATIONS ( BELOW 3.0 V
The LM4877 will function at voltages below 3 volts but this
mode of operation requires the addition of a 1k
from each of the differential output pins ( pins 8 and 4 ) di-
rectly to ground. The addition of the pair of 1k resistors ( R4
& R5 ) assures stable operation below 3 Volt Vdd operation.
The addition of the two resistors will however increase the
idle current by as much as 5mA. This is because at 0v input
both of the outputs of the LM4877’s 2 internal opamps go to
1/2 V
flow through the 1K resistors from output to ground. See fig
4.
Jumper options have been included on the reference design,
Fig. 4, to accommodate the low voltage application. J2 & J3
connect R4 and R5 to the outputs.
i.
B
A larger input coupling capacitor requires more charge to
equal to 0.1 µF, the device will be much more susceptible
DD
( 2.5 volts for a 5v power supply ), causing current to
B
B
, is the most critical component to minimize
equal to 1.0 µF along with a small value of C
DD
), the smaller the turn-on pop.
(Continued)
B
i
DD
DD
, forms a
equal to
resistor
). This
)
i
9
AUDIO POWER AMPLIFIER DESIGN
A 1W/8
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 output voltage. Using this method, the minimum
supply voltage would be (V
V
age vs Supply Voltage curve in the Typical Performance
Characteristics section.
Using the Output Power vs Supply Voltage graph for an 8
load, the minimum supply rail is 4.6V. But since 5V is a stan-
dard voltage in most applications, it is chosen for the supply
rail. Extra supply voltage creates headroom that allows the
LM4877 to reproduce peaks in excess of 1W without produc-
ing audible distortion. At this time, the designer must make
sure that the power supply choice along with the output im-
pedance does not violate the conditions explained in the
Power Dissipation section.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 3.
From Equation 3, the minimum A
Since the desired input impedance was 20 k , and with a
A
allocation of R
is to address the bandwidth requirements which must be
stated as a pair of −3 dB frequency points. Five times away
from a −3 dB point is 0.17 dB down from passband response
which is better than the required
As stated in the External Components section, R
junction with C
The high frequency pole is determined by the product of the
desired frequency pole, f
With a A
150 kHz which is much smaller than the LM4877 GBWP of
4 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4877 can still be used without running into bandwidth limi-
tations.
Given:
OD BOT
VD
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
f
f
C
impedance of 2, a ratio of 1.5:1 of R
L
H
i
= 100 Hz/5 = 20 Hz
= 20 kHz * 5 = 100 kHz
and V
VD
1/(2 *20 k *20 Hz) = 0.397 µF; use 0.39 µF
AUDIO AMPLIFIER
= 3 and f
OD TOP
i
i
= 20 k and R
create a highpass filter.
are extrapolated from the Dropout Volt-
R
H
f
/R
= 100 kHz, the resulting GBWP =
i
H
opeak
= A
, and the differential gain, A
f
100 Hz–20 kHz
VD
= 30 k . The final design step
+ (V
/2
VD
±
0.25 dB specified.
OD TOP
is 2.83; use A
opeak
f
+ V
to R
using Equation 2
OD BOT
i
±
results in an
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1 Wrms
0.25 dB
1 Vrms
VD
20 k
)), where
i
in con-
= 3.
8
VD
(2)
(3)
.