LM4668 National Semiconductor, LM4668 Datasheet - Page 8

• Manufacturer Part Number: LM4668
• Description: 10W High-Efficiency Mono BTL Audio Power Amplifier
• Manufacturer: National Semiconductor

www.national.com
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 LM4668 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 and sets
the nominal switching frequency at 450kHz. Modulation re-
sults 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 LM4668 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 LM4668’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 LM4668’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 deacti-
vates the output switching waveform and minimizes the
quiescent current. Applying logic 0 (GND) to pin 8 enables
the shutdown function. Applying logic 1 (4V ≤ V
to pin 8 disables the shutdown function and restores full
amplifier operation.
Under Voltage Protection
The under voltage protection disables the output driver sec-
tion of the LM4668 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 LM4668’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.
Turn-On Time
The LM4668 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.
DD
/2. Further, during turn-on, the outputs
LOGIC
≤ V
DD
)
8
Output Stage Fault Detection And Protection
The output stage MOSFETs are protected against output
conditions that could otherwise compromise their operational
status. An onboard fault detection circuit continuously moni-
tors the signal on each output MOSFET’s gate and com-
pares it against the respective drain voltage. When a condi-
tion 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 pro-
tective condition for approximately 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 LM4668 has thermal shutdown circuitry that monitors
the die temperature. Once the LM4668 die temperature
reaches 170˚C, the LM4668 disables the output switching
waveform and remains disabled until the die temperature
falls below 140˚C (typ).
Over-Modulation Protection
The LM4668’s over-modulation protection is a result of the
preamplifier’s (AMP1 and AMP2, Figure 1) inability to pro-
duce signal magnitudes that equal the power supply volt-
ages. Since the preamplifier’s output magnitude will always
be less than the supply voltage, the duty cycle of the ampli-
fier’s switching output will never reach zero. Peak modula-
tion is limited to a nominal 95%.
Application Hints
SUPPLY BYPASSING
Correct power supply bypassing has two important goals.
The first is to reduce noise on the power supply lines and
minimize deleterious effects that the noise may cause to the
amplifier’s operation. The second is to help stabilize an
unregulated power supply and to improve the supply’s tran-
sient response under heavy current demands. These two
goals require different capacitor value ranges. Therefore,
various types and values are recommended for supply by-
passing. For noise de-coupling, generally small ceramic ca-
pacitors (0.01µF to 0.1µF) are recommended. Larger value
(1µF to 10µF) tantalum capacitors are needed for the tran-
sient current demands. These two capacitors in parallel will
do an adequate job of removing most noise from the supply
rails and providing the necessary transient current. These
capacitors should be placed as close as possible to each
IC’s supply pin(s) using leads as short as possible.
The LM4668 has two V
signal V
ceramic (0.1µF) and high value tantalum (10µF) should be
used to bypass the PV
or tantalum can be used to bypass the SV
AMPLIFIER OUTPUT FILTERING
The LM4668 requires a lowpass filter connected between
the amplifier’s bridge output and the load. The second-order
LC output filter shown in Figure 1 creates the lowpass re-
sponse that is necessary to attenuate signal energy at the
amplifier’s switching frequency. It also serves to suppress
EMI. Together, the output filter’s 0.27µF capacitors and the
recommended minimum inductor value of 27µH produce a
DD
(SV
DD
). The parallel combination of the low value
DD
DD
pin. A small value (0.1µF) ceramic
pins: a power V
DD
DD
pin.
(PV
DD
) and a