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LM4884 Datasheet(Fiches technique) 17 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Numéro de pièce LM4884
Description  2.1W Differential Input, BTL Output Stereo Audio Amplifier with RF Suppression and Shutdown
Télécharger  22 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
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 17 page
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Application Information (Continued)
Table 2. Logic Level Truth Table for Shutdown
Operation
SHUTDOWN
OPERATIONAL
MODE
High
Full Power, stereo
BTL amplifiers
Low
Micro-power
Shutdown
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4884’s performance requires properly se-
lecting external components. Though the LM4884 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues. The LM4884 is unity-gain stable, giving a designer
maximum design flexibility. The gain should be set to no
more than a given application requires. This allows the am-
plifier to achieve minimum THD+N and maximum signal-to-
noise ratio. These parameters are compromised as the
closed-loop gain increases. However, low gain demands
input signals with greater voltage swings to achieve maxi-
mum output power. Fortunately, many signal sources such
as audio CODECs have outputs of 1V
RMS
(2.83V
P-P).
Please refer to the Audio Power Amplifier Design section for
more information on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (C
I,C2 and C3,C4) in Figure 1. A
high value capacitor can be expensive and may compromise
space efficiency in portable designs. In many cases, how-
ever, the speakers used in portable systems, whether inter-
nal or external, have little ability to reproduce signals with
frequencies below 150Hz. Applications using speakers with
this limited frequency response reap little improvement by
using large input capacitor.
Besides effecting system cost and size, CI - C4 can also
affect on the LM4884’s turn-on and turn-off transient ("click
and pop") performance. When the supply voltage is first
applied, a transient may be created as the charge on the
input capacitor changes from zero to a quiescent state. The
magnitude of the transient is proportional to the value of, and
more importantly, the mismatch between, the capacitors
connected to a given pair of inverting and non-inverting
inputs. The better the match, the less the transient magni-
tude.
Higher value capacitors need more time to reach a quiescent
DC voltage (usually V
DD/2) when charged with a fixed cur-
rent. This fixed current is supplied through amplifiers input
pins. Thus, selecting an input capacitor value that is no
higher than necessary to meet the desired -3dB frequency
will reduce turn-on time and help ensure that transients are
minimized.
The LM4884’s nominal input resistance (R
i)is25k
Ω (20kΩ,
minimum) and the input capacitor, C
i, form high pass filter
with a -3dB low frequency limit defined by equation (5).
f
-3dB = 1/2
π(25kΩ)C
i
(5)
As an example when using a speaker with a low frequency
limit of 150Hz, C
I, is 0.047µF. The 0.47µF CI shown in Figure
1 allows the LM4884 to drive high efficiency, full range
speaker whose response extends below 30Hz.
Bypass Capacitor Value Selection
Besides optimizing the input capacitor value, careful consid-
eration should be paid to value of C
B, the capacitor con-
nected between the BYPASS pin and ground. Since C
B
determines how fast the LM4884 settles to its quiescent
operating state, its value is critical when minimizing turn-on
transients. The slower the LM4884’s outputs ramp to their
quiescent DC voltage (nominally 12 V
DD), the smaller the
turn-on transient. Choosing C
B equal to 0.47µF along with a
small value of C
i (in the range of 0.047µF to 0.47µF), pro-
duces a transient-free turn-on and shutdown function. As
discussed above, choosing C
i no larger than necessary for
the desired bandwidth helps minimize turn-on transients.
OPTIMIZING OUTPUT TRANSIENT REDUCTION (CLICK
AND POP PERFORMANCE)
The LM4884 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 LM4884’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
DD. As soon as the
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
B alters the device’s turn-on
time and the magnitude of output transients. Increasing the
value of C
B reduces the magnitude of turn-on transients.
However, this presents a tradeoff: as the size of C
B in-
creases, the turn-on time increases. There is a linear rela-
tionships between the size of C
B + 2(CI) and the turn-on
time. The table shows some typical turn-on times for various
values of C
B:
C
B
Ton
C
i = 0.47µF
C
i = 0.33µF
0.01µF
110ms
80ms
0.1µF
120ms
90ms
0.22µF
140ms
100ms
0.47µF
170ms
140ms
1.0µF
240ms
210ms
In order to eliminate ’clicks and pops’, all capacitors must be
discharged before turn-on. Rapidly switching V
DD 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
Ω Load
The following are the desired operational parameters:
Power Output:
1 W
RMS
Load Impedance:
8
www.national.com
17




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