Datasheet

LM4853
www.ti.com SNAS155E –JANUARY 2002–REVISED MAY 2013
Selecting Input and Output Capacitor Values
Besides gain, one of the major considerations is the closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components shown in Figure 3. The input coupling capacitor C
I
and resistor R
I
form a first order high pass filter that limits low frequency response. C
I
's value should be based on
the desired frequency response weighed against the following: Large value input and output capacitors are both
expensive and space consuming for portable designs. Clearly a certain sized capacitor is needed to couple in
low frequencies without severe attenuation. But in many cases the speakers used in portable systems, whether
internal or external, have little ability to reproduce signals below 150Hz. Thus, large value input and output
capacitors may not increase system performance.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W / 8Ω Bridged Audio Amplifier
Given:
• Power Output: 1W
RMS
• Load Impedance 8Ω
• Input Level: 1V
RMS
• Input Impedance: 20kΩ
• Bandwidth: 100Hz - 20kHz ± 0.25dB
A designer must first determine the minimum supply voltage needed to obtain the specified output power. By
extrapolating from the Output Power vs Supply Voltage graphs in the Typical Performance Characteristics
section, the supply rail can be easily found. A second way to determine the minimum supply rail is to calculate
the required V
OPEAK
using Equation 5 and add the dropout voltage. This results in Equation 6, where V
ODTOP
and
V
ODBOT
are extrapolated from the Dropout Voltage vs Supply Voltage curve in the Typical Performance
Characteristics section.
(5)
V
DD
≥ (V
OPEAK
+ (V
ODTOP
+ V
ODBOT
)) (6)
Using the Output Power vs Supply Voltage graph for an 8Ω load, the minimum supply rail is 4.7V. But since 5V is
a standard supply voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates
headroom that allows the LM4853 to reproduce peaks in excess of 1W without producing audible distortion.
However, the designer must make sure that the chosen power supply voltage and output load 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 Equation 7.
(7)
R
F
/ R
I
= A
VD
/ 2 (8)
From Equation 6, the minimum A
VD
is 2.83; use A
VD
= 3.
The desired input impedance was 20kΩ, and with an A
VD
of 3, using Equation 8 results in an allocation of R
I
=
20kΩ and R
F
= 30kΩ.
The final design step is to set the amplifier's −3dB frequency bandwidth. To achieve the desired ± 0.25dB pass
band magnitude variation limit, the low frequency response must extend to at least one−fifth the lower bandwidth
limit and the high frequency response must extend o at least five times the upper bandwidth limit. The variation
for both response limits is 0.17dB, well within the ± 0.25dB desired limit. This results in:
f
L
= 100Hz / 5 = 20Hz (9)
f
H
= 20kHz x 5 = 100kHz (10)
As stated in the External Components Description section, R
I
in conjunction with C
I
create a highpass filter. Find
the coupling capacitor's value using Equation 11.
C
I
≥ 1 / (2πR
I
f
L
) (11)
C
I
≥ 1 / ( 2π × 20kΩ × 20Hz) = 0.397µF (12)
Use a 0.39µF capacitor, the closest standard value.
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