lm4651 National Semiconductor Corporation, lm4651 Datasheet - Page 10

• Manufacturer Part Number: lm4651
• Description: Overture? Audio Power Amplifier 170w Class D Audio Power Amplifier Solution
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
GENERAL FEATURES
System Functional Information: The LM4651 is a conven-
tional pulse width modulator/driver. As Figure 2 shows the
incoming audio signal is compared with a triangle waveform
with a much higher frequency than the audio signal (not
drawn to scale). The comparator creates a variable duty
cycle squarewave. The squarewave has a duty cycle propor-
tional to the audio signal level. The squarewave is then
properly conditioned to drive the gates of power MOSFETs in
an H-bridge configuration, such as the LM4652. The pulse
train of the power MOSFETs are then fed into a low pass
filter (usually a LC) which removes the high frequency and
delivers an amplified replica of the audio input signal to the
load.
Standby Function: The standby function of the LM4651 is
CMOS compatible, allowing the user to perform a muting of
the music as well as turning off all power MOSFETs by
shutting down the pulse width waveform. Standby has the
added advantage of minimizing the quiescent current. Be-
cause standby shuts down the pulse width waveform, the
attenuation of the music is complete (
mized, and any output noise is eliminated since there is no
modulation waveform. By placing a logic ’1’ or 5V at pin 13,
the standby function will be enabled. A logic ’0’ or 0V at pin
13 will disable the standby function allowing modulation by
the input signal.
Under Voltage Protection: The under voltage protection
disables the output driver section of the LM4651 while the
supply voltage is below
power is first applied or when low line, changes in load
resistance or power supply sag occurs. The under voltage
protection ensures that all power MOSFETs are off, eliminat-
ing any shoot-through current and minimizing pops or clicks
during turn-on and turn-off. The under voltage protection
gives the digital logic time to stabilize into known states
providing a popless turn on.
Start Up Sequence and Self-Diagnostic Timing: The
LM4651 has an internal soft start feature (see Figure 3) that
ensures reliable and consistent start-up while minimizing
turn-on thumps or pops. During the start-up cycle the system
is in standby mode. This start-up time is controlled externally
by adjusting the capacitance (C
the START pin. The start-up time can be controlled by the
capacitor value connected to the START pin given by Equa-
tion (1) or (2):
FIGURE 2. Conventional Pulse Width Modulation
t
START
C
START
= (8.4x10
= T
START
±
10.5V. This condition can occur as
4
)C
/(8.5x10
START
START
4
)
>
) value connected to
120dB), EMI is mini-
(seconds)
(Farads)
DS101277-1
(1)
(2)
10
The value of C
though the start-up sequence and the frequency that the
diagnostic circuitry checks to see if an error condition has
been corrected. An Error condition occurs if current limit,
thermal shut down, under voltage detection, or standby are
sensed. The self-diagnostic circuit checks to see if any one
of these error flags has been removed at a frequency set by
the C
10µF then the diagnostic circuitry will check approximately
every second to see if an error condition has been corrected.
If the error condition is no longer present, the LM4651/52 will
return to normal operation.
Current Limiting and Short Circuit Protection: The resis-
tor value connected between the SCKT pin and GND deter-
mines the maximum output current. Once the output current
is higher than the set limit, the short circuit protection turns
all power MOSFETs off. The current limit is set to a minimum
of 10A internally but can be increased by adjusting the value
of the R
This feature is designed to protect the MOSFETs by setting
the maximum output current limit under short circuit condi-
tions. It is designed to be a fail-safe protection when the
output terminals are shorted or a speaker fails and causes a
short circuit condition.
Thermal Protection The LM4651 has internal circuitry (pin
12) that is activated by the thermal shutdown output signal
from the LM4652 (pin 4). The LM4652 has thermal shut
down circuitry that monitors the temperature of the die. The
voltage on the TSD pin (pin 4 of the LM4652) goes high (6V)
once the temperature of the LM4652 die reaches 150˚C.
This pin should be connected directly to the TSD pin of the
LM4651 (pin 12). The LM4651 disables the pulse width
waveform when the LM4652 transmits the thermal shutdown
flag. The pulse width waveform remains disabled until the
TSD flag from the LM4652 goes low, signaling the junction
temperature has cooled to a safe level.
Dead Time Setting The DELAY pin on the LM4651 allows
the user to set the amount of dead time or break before
make of the system. This is the amount of time one pair of
FETs are off before another pair is switched on. Increased
dead time will reduce the shoot through current but has the
disadvantage of increasing THD. The dead time should be
START
I
SCKT
SCKT
FIGURE 3. Startup Timing Diagram
capacitor. For example, if the value of C
= 1X10
resistor. Equation (3) shows how to find R
START
5
/(10k \ R
sets the time it takes for the IC to go
SCKT
)
DS101277-70
(Amps)
START
(3)
SCKT
is
.