lm4897mm National Semiconductor Corporation, lm4897mm Datasheet - Page 11

• Manufacturer Part Number: lm4897mm
• Description: 1.1 Watt Audio Power Amplifier With Fade-in And Fade-out
• Manufacturer: National Semiconductor Corporation

Application Information
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
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 LM4897 turns
on. The slower the LM4897’s outputs ramp to their quiescent
DC voltage (nominally 1/2 V
Choosing C
the range of 0.1µF to 0.39µF), 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
1.0µF is recommended in all but the most cost sensitive
designs.
AUDIO POWER AMPLIFIER DESIGN
A 1W/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-
i
B
Given:
. A larger input coupling capacitor requires more charge to
equal to 0.1µF, the device will be much more susceptible
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
B
B
, is the most critical component to minimize
equal to 1.0µF along with a small value of C
DD
), the smaller the turn-on pop.
100Hz – 20kHz
(Continued)
B
±
1 Wrms
DD
1 Vrms
equal to
0.2 dB
20kΩ
). This
8Ω
i
(in
11
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.
5V is a standard voltage, in most applications, chosen for the
supply rail. Extra supply voltage creates headroom that al-
lows the LM4897 to reproduce peaks in excess of 1W with-
out producing audible distortion. 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 3.
From Equation 3, the minimum A
Since the desired input impedance was 20kΩ, and with a
A
R
the 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
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 LM4897 GBWP of
10 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4897 can still be used without running into bandwidth
limitations.
OD BOT
VD
i
= 20kΩ and R
f
f
C
of 3, a ratio of 1.5:1 of R
L
H
i
= 100Hz / 5 = 20Hz
= 20kHz * 5 = 100kHz
≥ 1 / (2π*20kΩ*20Hz) = 0.397µF; use 0.39µF
and V
VD
= 3 and f
OD TOP
i
create a highpass filter.
f
= 30kΩ. The final design step is to address
±
A
0.25dB specified.
are extrapolated from the Dropout Volt-
VD
H
= (R
= 100kHz, the resulting GBWP =
H
opeak
, and the differential gain, A
f
f
/ R
to R
+ (V
i
) 2
VD
i
results in an allocation of
OD TOP
is 2.83; use A
opeak
+ V
using Equation 2
OD BOT
www.national.com
VD
)), where
i
in con-
= 3.
VD
(2)
(3)
.