LM4923LQBD National Semiconductor, LM4923LQBD Datasheet - Page 11

• Manufacturer Part Number: LM4923LQBD
• Description: BOARD EVALUATION LM4923LQ
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4923LQBD

• Amplifier Type: Class AB
• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 1.1W x 1 @ 8 Ohm
• Voltage - Supply: 2.3 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4923
• Lead Free Status / RoHS Status: Not applicable / Not applicable

nation of proper gain selection. When used in its typical
application as a fully differential power amplifier the LM4923
does not require input coupling capacitors for input sources
with DC common-mode voltages of less than V
lowable input common-mode voltage levels are actually a
function of V
tion 5:
Special care must be taken to match the values of the feed-
back resistors (R
the input resistors (R
more in front. Because of the balanced nature of differential
amplifiers, resistor matching differences can result in net DC
currents across the load. This DC current can increase power
consumption, internal IC power dissipation, reduce PSRR,
and possibly damaging the loudspeaker. The chart below
demonstrates this problem by showing the effects of differing
values between the feedback resistors while assuming that
the input resistors are perfectly matched. The results below
apply to the application circuit shown in Figure 1, and as-
sumes that V
coupled inputs tied to ground.
Similar results would occur if the input resistors were not
carefully matched. Adding input coupling capacitors in be-
tween the signal source and the input resistors will eliminate
this problem, however, to achieve best performance with min-
imum component count it is highly recommended that both
the feedback and input resistors matched to 1% tolerance or
better.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8Ω Audio Amplifier
Tolerance R
20%
10%
5%
1%
0%
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
V
CMi
< (V
0.8R
0.9R
0.95R 1.05R -0.125V
0.99R 1.01R -0.025V
R
DD
F1
DD
DD
, R
F1
-1.2)*((R
= 5V, R
i
, and R
and R
R
1.2R
1.1R
R
-R
F2
i1
F
and R
/ R
F2
f
f
+(R
L
) to each other as well as matching
V
-0.500V
-0.250V
0
and may be determined by Equa-
I
= 8Ω, and the system has DC
02
= A
i2
i
)/(R
) to each other (see Figure 1)
- V
VD
01
f
)-V
100Hz–20kHz ± 0.25dB
DD
*(R
i
/ 2R
I
62.5mA
31.25mA
15.63mA
3.125mA
0
LOAD
DD
f
)
. Exact al-
1Wrms
1Vrms
20kΩ
8Ω
(5)
(6)
11
A designer must first determine the minimum supply rail to
obtain the specified output power. The supply rail can easily
be found by extrapolating from the Output Power vs Supply
Voltage graphs in the Typical Performance Characteris-
tics section. A second way to determine the minimum supply
rail is to calculate the required V
add the dropout voltages. Using this method, the minimum
supply voltage is (Vopeak + (V
V
age vs Supply Voltage curve in the Typical Performance
Characteristics section.
Using the Output Power vs Supply Voltage graph for an 8Ω
load, the minimum supply rail just about 5V. Extra supply volt-
age creates headroom that allows the LM4923 to reproduce
peaks in excess of 1W without producing audible distortion.
At this time, the designer must make sure that the power sup-
ply 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 8.
From Equation 7, the minimum A
input impedance was 20kΩ, a ratio of 2.83:1 of R
in an allocation of R
60kΩ for both feedback resistors. The final design step is to
address the bandwidth requirement which must be stated as
a single -3dB frequency point. Five times away from a -3dB
point is 0.17dB down from passband response which is better
than the required ±0.25dB specified.
The high frequency pole is determined by the product of the
desired frequency pole, f
With a A
150kHz which is much smaller than the LM4923 GBWP of
10MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the LM4923
can still be used without running into bandwidth limitations.
DO BOT
and V
VD
= 2.83 and f
DO TOP
f
H
i
= 20kHz * 5 = 100kHz
= 20kΩ for both input resistors and R
are extrapolated from the Dropout Volt-
R
H
f
H
= 100kHz, the resulting GBWP =
/ R
, and the differential gain, A
i
= A
DO TOP
OPEAK
VD
VD
is 2.83. Since the desired
using Equation 7 and
+ (V
DO BOT
f
www.national.com
to R
)), where
i
results
VD
(7)
(8)
f
=
.