LM4681 National Semiconductor, LM4681 Datasheet - Page 13

• Manufacturer Part Number: LM4681
• Description: Stereo CLASS D Audio Power Amplifier
• Manufacturer: National Semiconductor

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 fre-
quency, the LM4681 uses a balanced differential floating
modulator. Oscillation is a result of injecting complimentary
currents onto the respective plates of a floating, on-die ca-
pacitor. The value of the floating capacitor and value of the
components in the modulator’s feedback network set the
nominal switching 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
amplifier 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 out-
put stage. The LM4681 is immune to this type of noise
because the modulator, the components that set its switch-
ing frequency, and even the load all float with respect to
ground.
The balanced modulator’s pulse width modulated output
drives the gates of the LM4681’s H-bridge configured output
power MOSFETs. The pulse-train present at the power
MOSFETs’ 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 LM4681’s digitally controlled shutdown function allows
the user to place the amplifier in a shutdown mode while the
system power supply remains active. Activating shutdown
deactivates the output switching waveform and minimizes
the quiescent current. Through the SPI/I
the Mode control register’s Bit 0 is used to control the
LM4681’s shutdown function. A logic “0” activates shutdown,
whereas a logic “1” returns the amplifier to its operational
quiescent state. When the power supply voltage is first ap-
plied, the LM4681 is operating in the shutdown mode. For
more information on the digital interface, see the section
titled “SPI/I
Mute Function
The LM4681’s digitally-controlled mute function allows the
user to place the amplifier outputs in a 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 their V
bias. Through the SPI/I
register’s Bit 1 is used to control the LM4681’s audio mute
function. A logic “0” activates mute, whereas a logic “1”
deactivates mute. When the power supply voltage is first
applied, the LM4681’s headphone and Class D amplifier
outputs are muted. More information on the digital interface
is found in the section titled “SPI/I
Stereo Headphone Amplifier
The LM4681’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
2
C Serial Digital Interface.”
2
C digital interface, the Mode control
2
C Serial Digital Interface.”
2
C digital interface,
DD
/2 operational
13
desirable to stop driving external speakers. This is easily
achieved by using the active low HPSEL input. As shown in
typical application schematic in Figure 1, with no head-
phones connected to the headphone jack the input voltage
applied to the HPSEL pin is a logic low. In this state, the
Class D amplifiers are active and able to drive external
speakers. When headphones are plugged into the head-
phone jack, the switch internal to the jack is opened. This
changes the voltage applied to the HPSEL pin to a logic
high, shutting off the LM4681’s Class D amplifiers.
Under Voltage Proctection
The under voltage protection disables the output driver sec-
tion of the LM4681 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 LM4681’s power MOSFETs are off. This action
eliminates shoot-through current and minimizes output tran-
sients during turn-on and turn-off. The under voltage protec-
tion gives the digital logic time to stabilize into known states,
further minimizing turn output transients.
Power Supply Sequencing
To ensure best performance, please observe the following
power-up sequence. The I
applied first. Do not send any data to the LM4681’s internal
registers until the V
LM4681’s power-on reset, which activates the amplifier’s
shutdown and mute. Once I
supply voltage. Prior to removing the two supply voltages,
activate shutdown and mute.
Turn-On Time
The LM4681 has an internal timer that determines the am-
plifier’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
are muted. This minimizes output transients that may occur
while the part settles into is 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 LM4681’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
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), 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.
DD
DD
/2. Further, during turn-on, the outputs
is applied. This takes advantage of the
2
2
CV
CV
DD
DD
supply voltage should be
is applied, apply the V
DD
www.national.com
or GND.
DD