lm4893ma National Semiconductor Corporation, lm4893ma Datasheet - Page 13

• Manufacturer Part Number: lm4893ma
• Description: 1.1 Watt Audio Power Amplifier
• 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 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 LM4893 turns
on. The slower the LM4893’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.
Figure 2 shows the LM4893’s turn-on characteristics when
coming out of shutdown mode. Trace B is the differential
output signal across a BTL 8Ω load. The LM4893’s active-
low SHUTDOWN pin is driven by the logic signal shown in
Trace A. Trace C is the Vo1- output signal and Trace D is the
Vo2+ output signal. A shown in Figure 2, the differential
output signal Trace B appears just as Trace A transitions
from logic low to logic high (turn-on condition).
shutdown signal (Trace A). The inverting output (Trace
C) and the non-inverting output (Trace D) are applied
i.
B
A larger input coupling capacitor requires more charge to
equal to 0.1 µF, the device will be much more susceptible
Differential output signal (Trace B) is devoid of
transients. The SHUTDOWN pin is driven by a
FIGURE 2. LM4893 Turn-on Characteristics
B
B
, is the most critical component to minimize
equal to 1.0 µF along with a small value of C
across an 8Ω BTL load.
DD
), the smaller the turn-on pop.
(Continued)
20038097
B
DD
equal to
). This
i
13
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-
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 LM4893 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 20 kΩ, and with a
A
R
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
300 kHz which is much smaller than the LM4893 GBWP of
10 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4893 can still be used without running into bandwidth
limitations.
Given:
OD BOT
VD
i
= 20 kΩ and R
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
f
f
C
of 3, 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
and V
VD
= 3 and f
OD TOP
i
create a highpass filter.
f
A
are extrapolated from the Dropout Volt-
= 30 kΩ. The final design step is to
VD
H
= 100 kHz, the resulting GBWP =
= (R
H
opeak
±
, and the differential gain, A
0.25 dB specified.
f
f
/R
to R
100 Hz–20 kHz
+ (V
i
) 2
VD
i
results in an allocation of
OD TOP
is 2.83; use A
opeak
+ V
using Equation 2
OD BOT
±
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0.25 dB
1 Wrms
VD
1 Vrms
)), where
i
20 kΩ
in con-
= 3.
8Ω
VD
(2)
(3)
.