lm4874mh National Semiconductor Corporation, lm4874mh Datasheet - Page 17

• Manufacturer Part Number: lm4874mh
• Description: 2.1w Differential Input, Btl Output Stereo Audio Amplifier With Selectable Gain And Shutdown
• Manufacturer: National Semiconductor Corporation

Application Information
Bypass Capacitor Value Selection
Besides optimizing the input capacitor value, careful consid-
eration should be paid to value of C
nected between the BYPASS pin and ground. Since C
determines how fast the LM4874 settles to its quiescent
operating state, its value is critical when minimizing turn-on
transients. The slower the LM4874’s outputs ramp to their
quiescent DC voltage (nominally
turn-on transient. Choosing C
small value of C
duces a transient-free turn-on and shutdown function. As
discussed above, choosing C
the desired bandwidth helps minimize turn-on transients.
OPTIMIZING OUTPUT TRANSIENT REDUCTION (CLICK
AND POP PERFORMANCE)
The LM4874 contains circuitry to minimize turn-on and shut-
down 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. While the power
supply voltage is ramping to its final value, the LM4874’s
internal amplifiers are configured as unity gain buffers. An
internal current source changes the voltage of the BYPASS
pin in a controlled, linear manner. Ideally, the amplifier inputs
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 can not be
modified, changing the size of C
time and the magnitude of output transients. Increasing the
value of C
However, this presents a tradeoff: as the size of C
creases, the turn-on time increases. There is a linear rela-
tionships between the size of C
time. The table shows some typical turn-on times for various
values of C
In order to eliminate ’clicks and pops’, all capacitors must be
discharged before turn-on. Rapidly switching V
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:
Power Output:
Load Impedance:
Input Level:
Input Impedance:
Bandwidth:
C
0.01µF
0.22µF
0.47µF
0.1µF
1.0µF
B
B
B
:
reduces the magnitude of turn-on transients.
i
(in the range of 0.047µF to 0.47µF), pro-
C
i
120ms
140ms
170ms
240ms
110ms
= 0.47µF
B
i
no larger than necessary for
equal to 0.47µF along with a
B
100 Hz−20 kHz
B
alters the device’s turn-on
1
⁄
+ 2(C
2
Ton
V
B
DD
, the capacitor con-
DD
I
), the smaller the
) and the turn-on
(Continued)
C
. As soon as the
i
100ms
140ms
210ms
= 0.33µF
80ms
90ms
DD
±
1 W
0.25 dB
may not
1 V
20 kΩ
B
RMS
RMS
8Ω
in-
B
17
The design begins by specifying the minimum supply voltage
necessary to obtain the desired 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 (6), 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 (6). The result is
Equation (7).
The Output Power vs Supply Voltage graph for an 8Ω load
indicates a minimum supply voltage of 4.6V. This is easily
met by the commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the
LM4874 to produce peak output power in excess of 1W
without clipping or other audible distortion. The choice of
supply voltage must also not create a situation that violates
of maximum power dissipation as explained above in the
Power Dissipation section.
After satisfying the LM4874’s power dissipation require-
ments, the minimum differential gain is found using Equation
(8).
Thus, a minimum gain of 2.83 allows the LM4874’s to reach
full output swing and maintain low noise and THD+N perfor-
mance. For this example, let A
the gain will be set to 10dB (A
low to GAIN 0 and a logic high to GAIN 1.
The last step in this design example is setting the amplifier’s
-3dB frequency bandwidth. To achieve the desired
pass band magnitude variation limit, the low frequency re-
sponse must extend to at least one-fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. This extended bandwith
produces a gain variation of -0.17dB at the bandwith’s limits,
well within the
and an
V
DD
±
≥ (V
0.25dB desired limit. The results are an
f
H
f
OUTPEAK
= 20kHz x 5 = 100kHz
L
= 100Hz/5 = 20Hz
+ (V
VD
VD
OD TOP
= 3. In the example design,
= 3.2) by applying a logic
+ V
OD BOT
))
www.national.com
±
0.25dB
(10)
(6)
(7)
(8)
(9)