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LM4879SD Datasheet

  • LM4879SD

  • National Semiconductor [1.1 Watt Audio Power Amplifier]

  • 788.33KB

  • NSC

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LM4879
Application Information
Selection Of Input Capacitor Size
(Continued)
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenu-
ation. But in many cases the speakers used in portable
systems, whether internal or external, have little ability to
reproduce signals below 100 Hz to 150 Hz. Thus, using a
large input capacitor may not increase actual system perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance 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 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 consid-
eration 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 LM4879 turns
on. The slower the LM4879鈥檚 outputs ramp to their quiescent
DC voltage (nominally 1/2 V
DD
), the smaller the turn-on pop.
Choosing C
B
equal to 1.0 碌F along with a small value of C
i
(in the range of 0.1 碌F to 0.39 碌F), should produce a virtually
clickless and popless shutdown function. While the device
will function properly, (no oscillations or motorboating), with
C
B
equal to 0.1 碌F, the device will be much more susceptible
to turn-on clicks and pops. Thus, a value of C
B
equal to
1.0 碌F is recommended in all but the most cost sensitive
designs.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8鈩?Audio Amplifier
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
1 Wrms
8鈩?/div>
1 Vrms
20 k鈩?/div>
100 Hz鈥?0 kHz
卤
0.25 dB
formance Characteristics
section, the supply rail can be
easily found. A second way to determine the minimum sup-
ply 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
ODTOP
+ V
ODBOT
)), where
V
ODBOT
and V
ODTOP
are extrapolated from the Dropout Volt-
age vs Supply Voltage curve in the
Typical Performance
Characteristics
section.
(2)
5V is a standard voltage, in most applications, chosen for the
supply rail. Extra supply voltage creates headroom that al-
lows the LM4879 to reproduce peaks in excess of 1W with-
out 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 Equa-
tion 3.
(3)
A
VD
= (R
f
/R
i
) 2
From Equation 3, the minimum A
VD
is 2.83; use A
VD
= 3.
Since the desired input impedance was 20 k鈩? and with a
A
VD
of 3, a ratio of 1.5:1 of R
f
to R
i
results in an allocation of
R
i
= 20 k鈩?and R
f
= 30 k鈩? The final design step is to
address the bandwidth requirements which must be stated
as a pair of 鈭? dB frequency points. Five times away from a
鈭? dB point is 0.17 dB down from passband response which
is better than the required
卤
0.25 dB specified.
f
L
= 100 Hz/5 = 20 Hz
f
H
= 20 kHz * 5 = 100 kHz
As stated in the
External Components
section, R
i
in con-
junction with C
i
create a highpass filter.
C
i
鈮?/div>
1/(2蟺*20 k鈩?20 Hz) = 0.397 碌F; use 0.39 碌F
The high frequency pole is determined by the product of the
desired frequency pole, f
H
, and the differential gain, A
VD
.
With a A
VD
= 3 and f
H
= 100 kHz, the resulting GBWP =
300 kHz which is much smaller than the LM4879 GBWP of
10 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4879 can still be used without running into bandwidth
limitations.
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 Per-
13
www.national.com

LM4879SD 产品属性

  • National Semiconductor (TI)

  • Class-AB

  • 1.1 W x 1 at 8 Ohms

  • 1-Channel Mono

  • 0.1 %

  • 3 V, 5 V

  • + 85 C

  • LLP EP

  • Reel

  • 8 Ohms

  • Single

  • - 40 C

  • Differential

  • Single

  • 5.5 V

  • 2.2 V

LM4879SD相关型号PDF文件下载

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    Dual 105mW Headphone Amplifier with Digital Volume
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  • LM4818
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  • LM4819
    英文版
    350mW Audio Power Amplifier with Shutdown Mode
    NSC
    LM4819
  • LM4819
    英文版
    350mW Audio Power Amplifier with Shutdown Mode
    NATIONAL
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