Datasheet

LM4890
www.ti.com SNAS138L –SEPTEMBER 2001–REVISED MAY 2013
In addition to system cost and size, click and pop performance is effected by the size of the input coupling
capacitor, C
IN
. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage
(nominally 1/2 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
BYPASS
, is the most critical component to minimize turn-on pops since it determines how fast
the LM4890 turns on. The slower the LM4890's outputs ramp to their quiescent DC voltage (nominally 1/2V
DD
),
the smaller the turn-on pop. Choosing C
BYPASS
equal to 1.0µF along with a small value of C
IN
, (in the range of
0.1µF to 0.39µF), should produce a virtually clickless and popless shutdown function. While the device will
function properly, (no oscillations or motorboating), with C
BYPASS
equal to 0.1µF, the device will be much more
susceptible to turn-on clicks and pops. Thus, a value of C
BYPASS
equal to 1.0µF is recommended in all but the
most cost sensitive designs.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω Audio Amplifier
Given:
Power Output 1 Wrms
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
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 2 and add the output voltage. Using this method, the minimum supply voltage would be (V
opeak
+
(V
OD
TOP
+ V
OD
BOT
)), where V
OD
BOT
and V
OD
TOP
are extrapolated from the Dropout Voltage vs Supply Voltage curve in
theTypical Performance Characteristics.
(2)
5V is a standard voltage which in most applications is chosen for the supply rail. Extra supply voltage creates
headroom that allows the LM4890 to reproduce peaks in excess of 1W 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 the POWER DISSIPATION section.
Once the power dissipation equations have been addressed, the required differential gain can be determined
from Equation 3.
(3)
R
f
/R
IN
= A
VD
/2 (4)
From Equation 3, the minimum A
VD
is 2.83; use A
VD
= 3.
Since the desired input impedance is 20 kΩ, and with an A
VD
gain of 3, a ratio of 1.5:1 of R
f
to R
IN
results in an
allocation of R
IN
= 20 kΩ and R
f
= 30 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 −3 dB point is 0.17 dB down from
passband response which is better than the required ±0.25 dB specified.
f
L
= 100Hz/5 = 20Hz
f
H
= 20kHz * 5 = 100kHz
As stated in the External Components Description section, R
IN
in conjunction with C
IN
create a highpass filter.
C
IN
≥ 1/(2π*20 kΩ*20Hz) = 0.397µF; use 0.39µF
Copyright © 2001–2013, Texas Instruments Incorporated Submit Documentation Feedback 21
Product Folder Links: LM4890