參數(shù)資料
型號(hào)： LM4682SQ
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類(lèi)： 音頻／視頻放大
英文描述： 10 Watt Stereo CLASS D Audio Power Amplifier with Stereo Headphone Amplifier and DC Volume Control
中文描述： 10 W, 2 CHANNEL, AUDIO AMPLIFIER, QCC48
封裝： LLP-48
文件頁(yè)數(shù)： 5/22頁(yè)
文件大?。?/td> 1033K
代理商： LM4682SQ
General Features
SYSTEM FUNCTIONAL INFORMATION
Modulation Technique
Unlike typical Class D amplifiers that use single-ended com-
parators to generate a pulse-width modulated switching
waveform and RC timing circuits to set the switching frequen-
cy, the LM4682 uses a balanced differential floating modula-
tor. Oscillation is a result of injecting complimentary currents
onto the respective plates of a floating, on-die capacitor. The
value of the floating capacitor and value of the components
in the modulator’s feedback network set the nominal switch-
ing frequency at 450kHz. Modulation results from imbalances
in the injected currents. The amount of current imbalance is
directly proportional to the applied input signal’s magnitude
and frequency.
Using a balanced, floating modulator produces a Class D am-
plifier that is immune to common mode noise sources such
as substrate noise. This noise occurs because of the high
frequency, high current switching in the amplifier’s output
stage. The LM4682 is immune to this type of noise because
the modulator, the components that set its switching frequen-
cy, and even the load all float with respect to ground.
The balanced modulator’s pulse width modulated output
drives the gates of the LM4682’s H-bridge configured output
power MOSFETs. The pulse-train present at the power MOS-
FETs’ output is applied to an LC low pass filter that removes
the 450kHz energy component. The filter’s output signal,
which is applied to the driven load, is an amplified replica of
the audio input signal.
Shutdown Function
The LM4682’s active-low shutdown function allows the user
to place the amplifier in a shutdown mode while the system
power supply remains active. Activating shutdown stops the
output switching waveform and minimizes the quiescent cur-
rent. Applying logic “0” to SDB pin enables the shutdown
function. Applying logic “1” to SDB pin disables the shutdown
function and restores full amplifier operation.
Mute Function
The LM4682’s active-low mute function allows the user to
place the amplifier outputs in muted mode while the
amplifier’s analog input signals remain active. Activating mute
internally removes the analog input signal from the Class D
and headphone amplifier inputs. While muted the amplifier
inputs and outputs retain in their VDD/2 operational bias. Ap-
plying logic “0” to MUTEB pin activates mute. Applying logic
“1” to MUTEB pin deactivates mute. The MUTEB pin is pull-
down internally to put both Class D and headphone amplifier
outputs mute.
Stereo Headphone Amplifier
The LM4682’s stereo headphone amplifier operates continu-
ously, even while the Class D amplifiers are active. When
using headphones to listen to program material, it is usually
desirable to stop driving external speakers. This is easily
achieved by using the active high HPSEL input. As shown in
typical application schematic in Figure 1, with no headphones
connected to the headphone jack, the input voltage applied
to the HPSEL pin is a logic low. In this state, the Class D am-
plifiers are active and able to drive external speakers. When
headphones are plugged into the headphone jack, the switch
inside the jack is opened. This changes the voltage applied
to the HPSEL pin to a logic high, shutting off the LM4682’s
Class D amplifiers. The headphone control of the output con-
figuration is shown in Table 1.
TABLE 1. Headphone Controls
HP Sense Pin, HPSEL
Output Stage Configuration
0
Class D Amps Active
1
Class D Amps Inactive
Under Voltage Protection
The under voltage protection disables the output driver sec-
tion of the LM4682 while the supply voltage is below 8V. This
condition may occur as power is first applied or during low line
conditions, changes in load resistance, or when power supply
sag occurs. The under voltage protection ensures that all of
the LM4682’s power MOSFETs are off. This action eliminates
shoot-through current and minimizes output transients during
turn-on and turn-off. The under voltage protection gives the
digital logic time to stabilize into known states, further mini-
mizing turn on output transients.
Power Supply Sequencing
In order to stabilize LM4682 before any operation, a power-
up sequence for the power supplies is recommended. The
Power V
DD should be applied first. Without deactivating the
mute and shutdown function of the amplifiers, the VOLV
DD is
then applied. Prior to removing the two supply voltages, acti-
vate shutdown and mute.
Turn-On Time
The LM4682 has an internal timer that determines the
amplifier’s turn-on time. After power is first applied or the part
returns from shutdown, the nominal turn-on time is 600ms.
This delay allows all externally applied capacitors to charge
to a final value of V
DD/2. Further, during turn-on, the outputs
are muted. This minimizes output transients that may occur
while the part settles into its quiescent operating mode.
Output Stage Current Limit and Fault Detection
Protection
The output stage MOSFETs are protected against output
conditions that could otherwise compromise their operational
status. The first stage of protection is output current limiting.
When conditions that require high currents to drive a load, the
LM4682’s current limit circuitry clamps the output current at a
nominal value of 2.5A. The output waveform is present, but
may be clipped or its amplitude reduced. The same 2.5A
nominal current limit also occurs if the amplifier outputs are
shorted together or either output is shorted to V
DD or GND.
The second stage of protection is an onboard fault detection
circuit that continuously monitors the signal on each output
MOSFET’s gate and compares it against the respective drain
voltage. When a condition is detected that violates a
MOSFET’s Safe Operating Area (SOA) and the drive signal
is disconnected from the output MOSFETs’ gates. The fault
detect circuit maintains this protective condition for approxi-
mately 600ms, at which time the drive signal is reconnected.
If the fault condition is no longer present, normal operation
resumes. If the fault condition remains, however, the drive
signal is again disconnected.
Thermal Protection
The LM4682 has thermal shutdown circuitry that monitors the
die temperature. Once the LM4682 die temperature reaches
170°C, the LM4682 disables the output switching waveform
and remains disabled until the die temperature falls below
140°C (typ).
13
www.national.com
LM4682
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