參數(shù)資料
型號(hào)： LM4888SQ/NOPB
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 音頻／視頻放大
英文描述： 3 W, 2 CHANNEL, AUDIO AMPLIFIER, QCC24
封裝： LLP-24
文件頁(yè)數(shù)： 9/23頁(yè)
文件大?。?/td> 1181K
代理商： LM4888SQ/NOPB
Application Information (Continued)
150 Hz. Applications using speakers with this limited fre-
quency response reap little improvement by using large
input capacitor.
Besides effecting system cost and size, C1 and C2 have an
effect on the LM4888’s click and pop performance. When the
supply voltage is first applied, a transient (pop) is created as
the charge on the input capacitor changes from zero to a
quiescent state. The magnitude of the pop is directly propor-
tional to the input capacitor’s size. Higher value capacitors
need more time to reach a quiescent DC voltage (usually
V
DD/2) when charged with a fixed current. The amplifier’s
output charges the input capacitor through the feedback
resistors, R2 and R8. Thus, pops can be minimized by
selecting an input capacitor value that is no higher than
necessary to meet the desired 3dB frequency.
A shown in Figure 1, the input resistors (R1,4,5, and 6) and
the input capacitors, C1 and C2 produce a 3dB high pass
filter cutoff frequency that is found using Equation (7).
(7)
As an example when using a speaker with a low frequency
limit of 150Hz, C
1, using Equation (7) is 0.053F. The .33F
C
1 shown in Figure 1 allows the LM4888 to drive high
efficiency, full range speaker whose response extends below
30Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
6, the capacitor con-
nected to the BYPASS pin. Since C
6 determines how fast the
LM4888 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4888’s
outputs ramp to their quiescent DC voltage (nominally 1/2
V
DD), the smaller the turn-on pop. Choosing C6 equal to
1.0 F along with a small value of C
1 (in the range of 0.1 F
to 0.39 F), produces a click-less and pop-less shutdown
function. As discussed above, choosing C
1 no larger than
necessary for the desired bandwith helps minimize clicks
and pops. Connecting a 1F capacitor, C
6, between the
BYPASS pin and ground improves the internal bias voltage’s
stability and improves the amplifier’s PSRR.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4888 contains circuitry that minimizes turn-on and
shutdown transients or “clicks and pop”. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. When the part is
turned on, an internal current source changes the voltage of
the BYPASS pin in a controlled, linear manner. Ideally, the
input and outputs track the voltage applied to the BYPASS
pin. The gain of the internal amplifiers remains unity until the
voltage on the bypass pin reaches 1/2 V
DD. As soon as the
voltage on the bypass pin is stable, the device becomes fully
operational. Although the BYPASS pin current cannot be
modified, changing the size of C
6 alters the device’s turn-on
time and the magnitude of “clicks and pops”. Increasing the
value of C
6 reduces the magnitude of turn-on pops. How-
ever, this presents a tradeoff: as the size of C
6 increases, the
turn-on time increases. There is a linear relationship be-
tween the size of C
6 and the turn-on time. Here are some
typical turn-on times for various values of C
6:
C
6
T
ON
0.01F
30ms
0.1F
40ms
0.22F
60ms
0.47F
80ms
1.0F
140 ms
In order eliminate “clicks and pops”, all capacitors must be
discharged before turn-on. Rapidly switching V
DD on and off
may not allow the capacitors to fully discharge, which may
cause “clicks and pops”.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8
Load
The following are the desired operational parameters:
Power Output:
1W
RMS
Load Impedance:
8
Input Level:
1V
rms
Input Impedance:
20k
Bandwidth:
100Hz20kHz ± 0.25dB
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
find the minimum supply voltage is to use the Output Power
vs Supply Voltage curve in the Typical Performance Char-
acteristics section. Another way, using Equation (8), is to
calculate the peak output voltage necessary to achieve the
desired output power for a given load impedance. To ac-
count for the amplifier’s dropout voltage, two additional volt-
ages, based on the Dropout Voltage vs Supply Voltage in the
Typical Performance Characteristics curves, must be
added to the result obtained by Equation (8). The result in
Equation (9).
(8)
V
DD
≥ (V
OUTPEAK +(VODTOP +VODBOT))
(9)
The Output Power vs Supply Voltage graph for an 8
load
indicates a minimum supply voltage of 4.35V for a 1W output
at 1% THD+N. This is easily met by the commonly used 5V
supply voltage. The additional voltage creates the benefit of
headroom, allowing the LM4888 to produce peak output
power in excess of 1.3W at 5V of V
DD and 1% THD+N
without clipping or other audible distortion. The choice of
supply voltage must also not create a situation that violates
maximum power dissipation as explained above in the
Power Dissipation section.
After satisfying the LM4888’s power dissipation require-
ments, the minimum differential gain needed to achieve 1W
dissipation in an 8
load is found using Equation (10).
(10)
Thus, a minimum gain of 2.83 allows the LM4888’s to reach
full output swing and maintain low noise and THD+N perfor-
mance. For this example, let A
VD =3.
The amplifier’s overall gain (non 3D mode) is set using the
input (R1 and R9) and feedback resistors R2 and R8. With
the desired input impedance set at 20k
, the feedback
resistor is found using Equation (11).
R
2/R1 =AVD/2
(11)
LM4888
www.national.com
17
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