lm4863mtx National Semiconductor Corporation, lm4863mtx Datasheet - Page 14

• Manufacturer Part Number: lm4863mtx
• Description: Dual 2.2w Audio Amplifier Plus Stereo Headphone Function
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
phone jack contact pin from -OUTA and allows R1 to pull the
HP Sense pin up to V
tion, turns off Amp2A and Amp2B, and mutes the bridged
speaker. The amplifier then drives the headphones, whose
impedance is in parallel with resistor R2 and R3. These
resistors have negligible effect on the LM4863’s output drive
capability since the typical impedance of headphones is
32Ω.
Figure 2 also shows the suggested headphone jack electri-
cal connections. The jack is designed to mate with a three-
wire plug. The plug’s tip and ring should each carry one of
the two stereo output signals, whereas the sleeve should
carry the ground return. A headphone jack with one control
pin contact is sufficient to drive the HP-IN pin when connect-
ing headphones.
A microprocessor or a switch can replace the headphone
jack contact pin. When a microprocessor or switch applies a
voltage greater than 4V to the HP-IN pin, a bridge-connected
speaker is muted and Amp1A and Amp2A drive a pair of
headphones.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4863’s performance requires properly se-
lecting external components. Though the LM4863 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4863 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 amplifier to
achieve minimum THD+N and maximum signal-to-noise ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio CO-
DECs have outputs of 1V
Audio Power Amplifier Design section for more informa-
tion on selecting the proper gain.
FIGURE 2. Headphone Circuit
DD
. This enables the headphone func-
RMS
(2.83V
P-P
). Please refer to the
(Continued)
01288124
14
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C
tor can be expensive and may compromise space efficiency
in portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Applications
using speakers with this limited frequency response reap
little improvement by using large input capacitor.
Besides effecting system cost and size, C
the LM4863’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 quies-
cent state. The magnitude of the pop is directly proportional
to the input capacitor’s size. Higher value capacitors need
more time to reach a quiescent DC voltage (usually V
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
R
capacitor value that is no higher than necessary to meet the
desired -3dB frequency.
A shown in Figure 1, the input resistor (R
capacitor, C
that is found using Equation (7).
As an example when using a speaker with a low frequency
limit of 150Hz, C
C
ciency, 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
the LM4863 settles to quiescent operation, its value is critical
when minimizing turn−on pops. The slower the LM4863’s
outputs ramp to their quiescent DC voltage (nominally 1/2
V
1.0µF along with a small value of C
0.39µF), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C
sary for the desired bandwidth helps minimize clicks and
pops.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4863 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 is ramping to its final value, the LM4863’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 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
BYPASS pin is stable, the device becomes fully operational.
Although the bypass pin current cannot be modified, chang-
ing the size of C
magnitude of "clicks and pops". Increasing the value of C
reduces the magnitude of turn-on pops. However, this pre-
DD
f
I
. Thus, pops can be minimized by selecting an input
shown in Figure 1 allows the LM4863 to drive high effi-
), the smaller the turn−on pop. Choosing C
I
produce a −3dB high pass filter cutoff frequency
I
, using Equation (4), is 0.063µF. The 1.0µF
B
alters the device’s turn-on time and the
DD
i
in Figure 1). A high value capaci-
. As soon as the voltage on the
i
(in the range of 0.1µF to
B
i
B
no larger than neces-
determines how fast
, the capacitor con-
i
has an affect on
I
) and the input
B
equal to
DD
(7)
/2)
B