LM4904 National Semiconductor, LM4904 Datasheet - Page 12

• Manufacturer Part Number: LM4904
• Description: 1 Watt Audio Power Amplifier
• Manufacturer: National Semiconductor

www.national.com
Application Information
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.
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 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
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 LM4904 turns
on. The slower the LM4904’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
designs.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω Audio Amplifier
i.
B
Given:
A larger input coupling capacitor requires more charge to
equal to 0.1 µF, the device will be much more susceptible
Power Output
Load Impedance
Input Level
Input Impedance
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
1 Wrms
i
DD
, forms a
1 Vrms
equal to
20 kΩ
). This
8Ω
i
12
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.
5V is a standard voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4904 to reproduce peaks in excess of 1W
without producing audible distortion. At this time, the de-
signer 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 2.
From Equation 2, the minimum A
Since the desired input impedance was 20 kΩ, 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
300kHz which is much smaller than the LM4904 GBWP of
2.5MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4904 can still be used without running into bandwidth
limitations.
VD
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
≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF
VD
= 3 and f
i
i
= 20 kΩ and R
create a highpass filter.
R
H
f
/R
= 100 kHz, the resulting GBWP =
i
H
= A
, and the differential gain, A
f
VD
= 30 kΩ. The final design step
100 Hz–20 kHz
/2
VD
±
0.25 dB specified.
is 2.83; use A
f
to R
i
results in an
±
0.25 dB
VD
i
in con-
= 3.
VD
(2)
.