LM4861MX/NOPB National Semiconductor, LM4861MX/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM4861MX/NOPB
• Description: Audio Amp Speaker 1-CH Mono 1.1W Class-AB 8-Pin SOIC N T/R
• Manufacturer: National Semiconductor

Specifications of LM4861MX/NOPB

• Package: 8SOIC N
• Function: Speaker
• Amplifier Type: Class-AB
• Total Harmonic Distortion Noise: 0.72@8Ohm@1W %
• Typical Psrr: 65 dB
• Output Signal Type: Differential
• Maximum Load Resistance: 8 Ohm
• Output Type: 1-Channel Mono
• Amplifier Class: AB
• No. Of Channels: 1
• Output Power: 1.1W
• Supply Voltage Range: 2V To 5.5V
• Load Impedance: 8ohm
• Operating Temperature Range: -40°C To +85°C
• Amplifier Case Style: SOIC
• Rohs Compliant: Yes

Application Information
possible high frequency oscillations. Care should be taken
when calculating the −3dB frequency in that an incorrect
combination of R
typical combination of feedback resistor and capacitor that
will not produce audio band high frequency rolloff is R
100kΩ and C
point of approximately 320kHz. Once the differential gain of
the amplifier has been calculated, a choice of R
and C
External Components Description section.
VOICE-BAND AUDIO AMPLIFIER
Many applications, such as telephony, only require a voice-
band frequency response. Such an application usually re-
quires a flat frequency response from 300Hz to 3.5kHz. By
adjusting the component values of Figure 2, this common
application requirement can be implemented. The combina-
tion of R
lowpass filter. Using the typical voice-band frequency range,
with a passband differential gain of approximately 100, the
following values of R
tions stated in the External Components Description sec-
tion.
Five times away from a −3dB point is 0.17dB down from the
flatband response. With this selection of components, the
resulting −3dB points, f
spectively, resulting in a flatband frequency response of
better than
the passband. If a steeper rolloff is required, other common
bandpass filtering techniques can be used to achieve higher
order filters.
SINGLE-ENDED AUDIO AMPLIFIER
Although the typical application for the LM4861 is a bridged
monoaural amp, it can also be used to drive a load single-
endedly in applications, such as PC cards, which require that
one side of the load is tied to ground. Figure 3 shows a
common single-ended application, where V
drive the speaker. This output is coupled through a 470µF
capacitor, which blocks the half-supply DC bias that exists in
all single-supply amplifier configurations. This capacitor,
designated C
highpass filter. The −3dB point of this high pass filter is
1/(2πR
product of R
cies to the load. When driving an 8Ω load, and if a full audio
spectrum reproduction is required, C
470µF. V
a 0.1 µF capacitor to a 2kΩ load to prevent instability. While
such an instability will not affect the waveform of V
good design practice to load the second output.
R
f
i
L
can then be calculated from the formula stated in the
= 10kΩ, R
C
i
O2
O
and C
), so care should be taken to make sure that the
, the output that is not used, is connected through
±
L
0.25dB with a rolloff of 6dB/octave outside of
O
f
and C
= 5pF. These components result in a −3dB
i
in Figure 3, in conjunction with R
form a highpass filter while R
f
f
and C
= 510k ,C
O
i
, C
is large enough to pass low frequen-
L
f
i
, R
and f
will cause rolloff before 20kHz. A
f
, and C
i
= 0.22µF, and C
H
, are 72Hz and 20kHz, re-
f
follow from the equa-
O
should be at least
(Continued)
f
O1
and C
f
= 15pF
f
is used to
L
will result,
, forms a
f
O1
form a
, it is
f
=
9
AUDIO POWER AMPLIFIER DESIGN
Design a 1W / 8Ω Audio Amplifier
A designer must first determine the needed supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graph 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
typically 0.6V.
For 1W of output power into an 8Ω load, the required V
is 4.0V. A minumum supply rail of 4.6V results from adding
V
in many applications and for this reason, a supply rail of 5V
is designated. Extra supply voltage creates dynamic head-
room that allows the LM4861 to reproduce peaks in excess
of 1Wwithout 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.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 4.
From equation 4, the minimum A
Since the desired input impedance was 20kΩ, and with a A
of 3, a ratio of 1:1.5 of R
20kΩ, R
bandwidth requirements which must be stated as a pair of
−3dB frequency points. Five times away from a −3db point is
0.17dB down from passband response which is better than
the required
high frequency pole of 20Hz and 100kHz respectively. As
stated in the External Components section, R
tion with C
The high frequency pole is determined by the product of the
desired high frequency pole, f
With a A
100kHz which is much smaller than the LM4861 GBWP of
4MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4861 can still be used without running into bandwidth
problems.
Given:
opeak
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
C
and V
i
≥ 1 / (2π*20kΩ*20Hz) = 0.397µF; use 0.39µF.
f
vd
= 30kΩ. The final design step is to address the
i
create a highpass filter.
= 2 and f
±
od
0.25dB specified. This fact results in a low and
. But 4.6V is not a standard voltage that exists
R
f
/R
H
f
i
= 100kHz, the resulting GBWP =
to R
= A
VD
H
i
results in an allocation of R
, and the differential gain, A
100 Hz–20 kHz
/ 2
opeak
vd
is 2.83: A
opeak
+ V
OD
using Equation 3
, where V
vd
i
= 3
±
www.national.com
in conjunc-
0.25 dB
1 Wrms
1 Vrms
20 kΩ
OD
opeak
8Ω
(3)
(4)
(5)
vd
i
vd
is
=
.