lm4838mtx National Semiconductor Corporation, lm4838mtx Datasheet - Page 18

• Manufacturer Part Number: lm4838mtx
• Description: Stereo 2w Audio Power Amplifiers With Dc Volume Control And Selectable Gain
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
INPUT CAPACITOR VALUE SELECTION
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor (0.33µF in Figure 2), but high
value capacitors 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 below
150 Hz. Applications using speakers with this limited fre-
quency response reap little improvement by using a large
input capacitor.
Besides effecting system cost and size, the input coupling
capacitor has an affect on the LM4838’s click and pop per-
formance. When the supply voltage is first applied, a tran-
sient (pop) is created as the charge on the input capacitor
changes from zero to a quiescent 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
rent. The amplifier’s output charges the input capacitor
through the feedback resistor, R
mized by selecting an input capacitor value that is no higher
than necessary to meet the desired −6dB frequency.
As shown in Figure 2, the input resistor (R
the input capacitor (C
pass filter cutoff frequency that is found using Equation (7).
As an example when using a speaker with a low frequency
limit of 150Hz, the input coupling capacitor, using Equation
(7), is 0.053µF. The 0.33µF input coupling capacitor shown
in Figure 2 allows the LM4838 to drive a high efficiency, full
range speaker whose response extends below 30Hz.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4838 contains circuitry that minimizes turn-on and
shutdown transients or “clicks and pops”. For this discus-
sion, turn-on refers to either applying the power supply volt-
age or when the shutdown mode is deactivated. While the
power supply is ramping to its final value, the LM4838’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
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
B
reduces the magnitude of turn-on pops. How-
B
IR
and the turn-on time. Here are some
DD
, C
/2) when charged with a fixed cur-
IL
= 0.33µF) produce a −6dB high
B
f
. Thus, pops can be mini-
DD
alters the device’s turn-on
. As soon as the voltage
IR
(Continued)
, R
B
B
:
IL
increases, the
= 20k) ( and
(7)
18
DOCKING STATION INTERFACE
Applications such as notebook computers can take advan-
tage of a docking station to connect to external devices such
as monitors or audio/visual equipment that sends or receives
line level signals. The LM4838 has two outputs, Right Dock
and Left Dock, which connect to outputs of the internal input
amplifiers that drive the volume control inputs. These input
amplifiers can drive loads of
ers) with a rail-to-rail signal. Since the output signal present
on the RIGHT DOCK and LEFT DOCK pins is biased to
V
with the load when using these outputs. Typical values for
the output coupling capacitors are 0.33µF to 1.0µF. If polar-
ized coupling capacitors are used, connect their "+" termi-
nals to the respective output pin, see Figure 2.
Since the DOCK outputs precede the internal volume con-
trol, the signal amplitude will be equal to the input signal’s
magnitude and cannot be adjusted. However, the input am-
plifier’s closed-loop gain can be adjusted using external
resistors. These 20k resistors (R
ure 2 and they set each input amplifier’s gain to -1. Use
Equation 7 to determine the input and feedback resistor
values for a desired gain.
Adjusting the input amplifier’s gain sets the minimum gain for
that channel. Although the single ended output of the Bridge
Output Amplifiers can be used to drive line level outputs, it is
recommended that the R & L Dock Outputs simpler signal
path be used for better performance.
BEEP DETECT FUNCTION
Computers and notebooks produce a system “beep“ signal
that drives a small speaker. The speaker’s auditory output
signifies that the system requires user attention or input. To
accommodate this system alert signal, the LM4838’s beep
input pin is a mono input that accepts the beep signal.
Internal level detection circuitry at this input monitors the
beep signal’s magnitude. When a signal level greater than
V
amplifiers are enabled. The beep signal is amplified and
applied to the load connected to the output amplifiers. A valid
beep signal will be applied to the load even when MUTE is
active. Use the input resistors connected between the BEEP
IN pin and the stereo input pins to accommodate different
beep signal amplitudes. These resistors (R
as 200kΩ devices in Figure 2. Use higher value resistors to
reduce the gain applied to the beep signal. The resistors
must be used to pass the beep signal to the stereo inputs.
The BEEP IN pin is used only to detect the beep signal’s
magnitude: it does not pass the signal to the output amplifi-
ers. The LM4838’s shutdown mode must be deactivated
before a system alert signal is applied to BEEP IN pin.
DD
DD
/2, coupling capacitors should be connected in series
/2 is detected on the BEEP IN pin, the bridge output
- A
VR
= R
FR
C
0.01µF
0.22µF
0.47µF
0.1µF
1.0µF
/R
B
IR
>
and - A
1kΩ (such as powered speak-
FR
T
200ms
, R
ON
VL
20ms
44ms
94ms
2ms
FL
= R
) are shown in Fig-
FL
BEEP
/R
IL
) are shown
(8)