lm4888sqx National Semiconductor Corporation, lm4888sqx Datasheet - Page 17

• Manufacturer Part Number: lm4888sqx
• Description: Dual 2.1w Audio Amplifier Plus Stereo Headphone & 3d Enhancement
• Manufacturer: National Semiconductor Corporation

Application Information
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
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).
As an example when using a speaker with a low frequency
limit of 150Hz, C
C
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
nected to the BYPASS pin. Since C
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
1.0 µF along with a small value of C
to 0.39 µF), produces a click-less and pop-less shutdown
function. As discussed above, choosing C
necessary for the desired bandwith helps minimize clicks
and pops. Connecting a 1µF capacitor, C
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
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
time and the magnitude of “clicks and pops”. Increasing the
value of C
ever, this presents a tradeoff: as the size of C
turn-on time increases. There is a linear relationship be-
tween the size of C
typical turn-on times for various values of C
DD
DD
1
shown in Figure 1 allows the LM4888 to drive high
/2) when charged with a fixed current. The amplifier’s
), the smaller the turn-on pop. Choosing C
6
reduces the magnitude of turn-on pops. How-
1
, using Equation (7) is 0.053µF. The .33µF
6
and the turn-on time. Here are some
6
alters the device’s turn-on
6
1
determines how fast the
(in the range of 0.1 µF
6
, the capacitor con-
DD
(Continued)
1
. As soon as the
6
6
6
:
no larger than
, between the
increases, the
6
equal to
(7)
17
In order eliminate “clicks and pops”, all capacitors must be
discharged before turn-on. Rapidly switching V
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 8Ω Load
The following are the desired operational parameters:
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).
The Output Power vs Supply Voltage graph for an 8Ω 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
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 8Ω load is found using Equation (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
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 20kΩ, the feedback
resistor is found using Equation (11).
Power Output:
Load Impedance:
Input Level:
Input Impedance:
Bandwidth:
V
DD
≥ (V
OUTPEAK
C
0.01µF
0.22µF
0.47µF
R
0.1µF
1.0µF
6
2
/R
1
+ (V
= A
VD
OD TOP
VD
= 3.
T
140 ms
100Hz−20kHz
/2
ON
30ms
40ms
60ms
80ms
+ V
DD
OD BOT
and 1% THD+N
DD
))
www.national.com
±
on and off
1W
0.25dB
1V
20kΩ
RMS
(10)
8Ω
rms
(11)
(8)
(9)