=
*
*
Ci ≥ 1/(2π 20 kΩ 20 Hz) 0.397 µF; use 0.33 µF
Application Information (Continued)
The high frequency pole is determined by the product of the
desired high frequency pole, fH, and the differential gain, A
NO-LOAD DESIGN CONSIDERATIONS
=
=
=
If the outputs of the LM4863 have a load higher than 10kΩ,
the LM4863 may show a small oscillation at high output lev-
els. To prevent this oscillation, place 5kΩ resistors from the
power outputs to ground.
VD. With a AVD 3 and fH 100 kHz, the resulting GBWP
150 kHz which is much smaller than the LM4863 GBWP of
3.5 MHz. This figure displays that if a designer has a need to
design an amplifier with
a higher differential gain, the
LM4863 can still be used without running into bandwidth
problems.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8Ω Bridged Audio Amplifier
DEMOBOARD CIRCUIT LAYOUT
The demoboard circuit layout is provided here as an ex-
ample of a circuit using the LM4863. If an LM4863MTE is
used with this layout, the exposed-DAP is soldered down to
the copper pad beneath the part. Heat is conducted away
from the part by the two large copper pads in the upper cor-
ners of the demoboard.
Given:
Power Output:
Load Impedance:
Input Level:
1 Wrms
8Ω
1 Vrms
20 kΩ
Input Impedance:
Bandwidth:
This demoboard provides enough heat dissipation ability to
allow an LM4863MTE to output 2.2W into 4Ω at 25˚C.
±
100 Hz−20 kHz 0.25 dB
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 Vopeak using Equation 3
and add the dropout voltage. Using this method, the mini-
*
mum supply voltage would be (Vopeak + (2 Vod)), where Vod
is extrapolated from the Dropout Voltage vs Supply Voltage
curve in the Typical Performance Characteristics section.
(4)
Using the Output Power vs Supply Voltage graph for an 8Ω
load, the minimum supply rail is 3.9V. But since 5V is a stan-
dard supply voltage in most applications, it is chosen for the
supply rail. Extra supply voltage creates headroom that al-
lows the LM4863 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.
DS012881-94
All Layers
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 4.
(5)
=
Rf/R
AVD/2
(6)
3
i
=
From equation 4, the minimum AVD is 2.83; use AVD
Since the desired input impedance was 20 kΩ, and with a
AVD of 3, a ratio of 1.5:1 of Rf to Riresults in an allocation of
DS012881-93
=
=
Ri 20 kΩ and R
30 kΩ. The final design step is to ad-
f
Silk Screen Layer
dress the bandwidth requirements which must be stated as a
pair of −3 dB frequency points. Five times away from a pole
gives 0.17 dB down from passband response, which is better
±
than the required 0.25 dB specified.
=
=
fL 100 Hz/5 20 Hz
=
=
fH 20 kHz x 5 100 kHz
As stated in the External Components section, Ri in con-
junction with Ci create a highpass filter.
11
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