lm4651 National Semiconductor Corporation, lm4651 Datasheet - Page 11

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

Application Information
reduced as the desired bandwidth of operation increases.
The dead time can be adjusted with the R
Equation (4):
Currently, the recommended value is 5k .
Oscillator Control: The modulation frequency is set by an
external resistor, R
GND. The modulation frequency can be set within the range
of 50kHz to 225kHz according to the design requirements.
The values of R
and (6):
Equations (5) and (6) are for R
greater than zero will increase the value needed for R
For R
2k . As the graphs show, increasing the switching frequency
will reduce the THD but also decreases the efficiency and
maximum output power level before clipping. Increasing the
switching frequency increases the amount of loss because
switching currents lower the efficiency across the output
power range. A higher switching frequency also lowers the
maximum output power before clipping or the 1% THD point
occur.
Over-Modulation Protection: The over-modulation protec-
tion is an internally generated fixed pulse width signal that
prevents any side of the H-bridge power MOSFETs from
remaining active for an extended period of time. This condi-
tion can result when the input signal amplitude is higher than
the internal triangle waveform. Lack of an over modulation
signal can increase distortion when the amplifier’s output is
clipping. Figure 4 shows how the over modulation protection
works.
The over modulation protection also provides a ’soft clip’
type response on the top of a sine wave. This minimum
pulse time is internally set and cannot be adjusted. As the
switching frequency increases this minimum time becomes a
higher percentage of the period (T
the over modulation protection time is a higher percentage of
the period, the peak output voltage is lower and, therefore,
the output power at clipping is lower for the same given
supply rails and load.
T
DLY
DLY
R
f
OSC
OSC
FIGURE 4. Over Modulation Protection
= 1.7x10
= 5k , R
= 1x10
= (1x10
OSC
−12
OSC
OSC
and f
9
9
/ (4000 + R
/ f
(500 + R
, connected between pin 16 and
OSC
will need to be increased by about
OSC
) − 4000
DLY
can be found by Equation (5)
DLY
OSC
= 0. Using a value of R
PERIOD
)
)
(Seconds)
(Continued)
= 1/f
(Hz)
( )
DLY
DS101277-2
SW
resistor by
). Because
(5)
(6)
(4)
OSC
DLY
.
11
Feedback Amplifier and Filter: The purpose of the feed-
back amplifier is to differentially sample the output and pro-
vide a single-ended feedback signal to the error amplifier to
close the feedback loop. The feedback is taken directly from
the switching output before the demodulating LC filter to
avoid the phase shift caused by the output filter. The signal
fed back is first low pass filtered with a single pole or dual
pole RC filter to remove the switching frequency and its
harmonics. The differential signal, derived from the bridge
output, goes into the high input impedance instrumentation
amplifier that is used as the feedback amplifier. The instru-
mentation amplifier has an internally fixed gain of 1. The use
of an instrumentation amplifier serves two purposes. First,
it’s input are high impedance so it doesn’t load down the
output stage. Secondly, an IA has excellent common-mode
rejection when its gain setting resistors are properly
matched. This feature allows the IA to derive the true feed-
back signal from the differential output, which aids in improv-
ing the system performance.
Error Amplifier: The purpose of the error amplifier is to sum
the input audio signal with the feedback signal derived from
the output. This inverting amplifier’s gain is externally con-
figurable by resistors Rf and R1. The parallel feedback ca-
pacitor and resistor form a low pass filter that limits the
frequency content of the input audio signal and the feedback
signal. The pole of the filter is set by Equation (7).
On-Board Regulators: The LM4651 has its own internal
supply regulators for both analog and digital circuits. Sepa-
rate
oscillator and PWM comparators. A separate voltage regu-
lator powers the digital logic that controls the protection,
level shifting, and high−/low−side driver circuits. System per-
formance is enhanced by bypassing each regulator’s output.
The
−6V
regulator output, −V
passed to −V
should be always be 6V closer to ground than the negative
rail, −V
should equal −14V. Recommended capacitor values and
type can be found in Figure 1, Typical audio Application
Circuit.
APPLICATIONS HINTS
BYP
FIGURE 5. Feedback instrumentation Amplifier
±
±
6V regulators exist solely for the analog amplifiers,
6V regulator outputs, labeled +6V
EE
(pin 8) should be bypassed to ground. The digital
. As an example, if −V
EE
(pins 22 & 23). The voltage level of −V
f
IP
DDBYP
= 1/(2 R
Schematic
(pins 20 & 21) should be by-
f
C
f
EE
)
= −20V, then −V
(Hz)
DS101277-3
BYP
(7)
(pin 6) and
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DDBYP
DDBYP