LM4818M National Semiconductor, LM4818M Datasheet - Page 11

• Manufacturer Part Number: LM4818M
• Description: 350mW Audio Power Amplifier with Shutdown Mode
• Manufacturer: National Semiconductor

Application Information
In order to eliminate ’clicks and pops’, all capacitors must be
discharged before turn-on. Rapidly switching V
allow the capacitors to fully discharge, which may cause
’clicks and pops’.
AUDIO POWER AMPLIFIER DESIGN EXAMPLE
parameters:
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. To find this
minimum supply voltage, use the Output Power vs. Supply
Voltage graph in the Typical Performance Characteristics
section. From the graph for a 16
16 , and 32 loads) the supply voltage for 100mW of output
power with 1% THD+N is approximately 3.15 volts.
Additional supply voltage creates the benefit of increased
headroom that allows the LM4818 to reproduce peaks in
excess of 100mW without output signal clipping or audible
distortion. The choice of supply voltage must also not create
a situation that violates maximum dissipation as explained
above in the Power Dissipation section. For example, if a
3.3V supply is chosen for extra headroom then according to
Equation (3) the maximum power dissipation point with a
16
bient temperature is 126˚C for the M08A package.
After satisfying the LM4818’s power dissipation require-
ments, the minimum differential gain is found using Equation
(6).
Thus a minimum gain of 1.27 V/V allows the LM4818 to
reach full output swing and maintain low noise and THD+N
performance. For this example, let A
er’s overall gain is set using the input (R
resistors. With the desired input impedance set to 20k , the
feedback resistor is found using Equation (7).
The value of R
Given:
The following are the desired operational
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
load is 138mW. Using Equation (4) the maximum am-
F
is 13k .
R
F
/R
0.47µF
C
1.0µF
i
B
= A
VD
/2 (V/V)
100Hz–20kHz
T
940ms
load, (graphs are for 8 ,
ON
VD
2S
i
= 1.27. The amplifi-
) and feedback (R
(Continued)
1Vrms (max)
±
DD
100mW
0.25dB
may not
20k
16
(6)
(7)
F
)
11
The last step in this design example is setting the amplifier’s
-3dB frequency bandwidth. To achieve the desired
pass band magnitude variation limit, the low frequency re-
sponse must extend to at least one-fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. The gain variation for
both response limits is 0.17dB, well with in the
desired limit.
The results are:
As mentioned in the External Components section, R
C
band pass frequency limit. Find the coupling capacitor’s
value using Equation (8).
C
product of the desired high frequency cutoff (100kHz in this
example) and the differential gain, A
per pass band response limit. With A
100kHz, the closed-loop gain bandwidth product (GBWP) is
127kHz. This is less than the LM4818’s 900kHz GBWP. With
this margin the amplifier can be used in designs that require
more differential gain while avoiding performance restricting
bandwidth limitations.
i
i
create a high pass filter that sets the amplifier’s lower
f
f
0.398µF, a standard value of 0.39µF will be used. The
L
H
= 100Hz/5 = 20Hz
= 20 kHz*5 = 100kHz
C
i
1/(2 R
i
f
c
) (F)
VD
, determines the up-
VD
= 1.27 and f
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±
±
0.25dB
0.25dB
i
and
H
(8)
=