LM4938MH National Semiconductor, LM4938MH Datasheet - Page 17

• Manufacturer Part Number: LM4938MH
• Description: IC,Audio Amplifier,DUAL,TSSOP,28PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM4938MH

• No. Of Channels: 2
• Output Power: 2W
• Supply Voltage Range: 2.7V To 5.5V
• Thd + N: 0.05% @ 400mW, 8ohm, VDD=5V
• Load Impedance: 3ohm
• Operating Temperature Range: -20°C To +85°C
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Information
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 LM4938 to drive a high efficiency, full
range speaker whose response extends below 30Hz.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4938 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 LM4938’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
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 LM4938 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
B
reduces the magnitude of turn-on pops. How-
B
IR
and the turn-on time.
, C
IL
= 0.33µF) produce a −6dB high
B
f
DD
. Thus, pops can be mini-
alters the device’s turn-on
. As soon as the voltage
IR
(Continued)
, R
B
IL
increases, the
= 20k) ( and
(7)
17
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 LM4938’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 LM4938’s shutdown mode must be deactivated
before a system alert signal is applied to BEEP IN pin.
If the “Beep” feature is not needed, remove the two Beep
Resistors (200k) and Beep input capacitor (.33µf). Then, tie
the Beep input pin (#11) to ground. Note that the Beep
Circuit is designed to operate with only a square wave input
from a control source.
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
/R
IR
>
and - A
1kΩ (such as powered speak-
FR
, R
VL
FL
= R
) are shown in Fig-
FL
BEEP
/R
IL
) are shown
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(8)