Datasheet

LM4864
SNAS109F –SEPTEMBER 1999–REVISED MAY 2013 www.ti.com
In addition to system cost and size, click and pop performance is effected by the size of the input coupling
capacitor, C
i
. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally
½ V
DD
). This 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 consideration should be paid to the bypass capacitor value.
Bypass capacitor, C
B
, is the most critical component to minimize turn-on pops since it determines how fast the
LM4864 turns on. The slower the LM4864's outputs ramp to their quiescent DC voltage (nominally ½ V
DD
), the
smaller the turn-on pop. Choosing C
B
equal to 1.0 μF along with a small value of C
i
(in the range of 0.1 μF to
0.39 μF), should produce a clickless and popless shutdown function. While the device will function properly, (no
oscillations or motorboating), with C
B
equal to 0.1 μF, the device will be much more susceptible to turn-on clicks
and pops. Thus, a value of C
B
equal to 1.0 μF or larger is recommended in all but the most cost sensitive
designs.
AUDIO POWER AMPLIFIER DESIGN
Design a 300 mW/8Ω Audio Amplifier
Given:
Power Output 300 mWrms
Load Impedance 8Ω
Input Level 1 Vrms
Input Impedance 20 kΩ
Bandwidth 100 Hz–20 kHz ± 0.25 dB
A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating
fromFigure 18 and Figure 19 in Typical Performance Characteristics, 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. Using this method, the minimum supply voltage would be (V
opeak
+ (2*V
OD
)), where V
OD
is
extrapolated from Figure 23 in Typical Performance Characteristics.
(5)
Using Figure 17 for an 8Ω load, the minimum supply rail is 3.5V. 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 LM4864
to reproduce peaks in excess of 500 mW without 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 POWER DISSIPATION.
Once the power dissipation equations have been addressed, the required differential gain can be determined
from Equation 6.
(6)
R
F
/R
i
= A
VD
/2 (7)
From Equation 6, the minimum A
VD
is 1.55; use A
VD
= 2.
Since the desired input impedance was 20 kΩ, and with a A
VD
of 2, a ratio of 1:1 of R
F
to R
i
results in an
allocation of R
i
= R
F
= 20 kΩ. The final design step is to address 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.
f
L
= 100 Hz/5 = 20 Hz (8)
f
H
= 20 kHz × 5 = 100 kHz (9)
As stated in External Components Description , R
i
in conjunction with C
i
create a highpass filter.
(10)
C
i
≥ 1/(2π*20 kΩ*20 Hz) = 0.397 μF; use 0.39 μF (11)
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