lm4651 National Semiconductor Corporation, lm4651 Datasheet - Page 13

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

Application Information
applications. In bridge operation, each output sees C
This causes the extra factor of 2 in the formula. The alterna-
tive to C
V
size or cost efficient because each capacitor must be twice
C
additional small value capacitors connected between each
output and ground (C
output to ground . The recommended value for C
1µF or 2% to 20% of C
Modulation Frequency Optimization
Setting the modulation frequency depends largely on the
application requirements. To maximize efficiency and output
power a lower modulation frequency should be used. The
lower modulation frequency will lower the amount of loss
caused by switching the output MOSFETs increasing the
efficiency a few percent. A lower switching frequency will
also increase the peak output power before clipping because
the over modulation protection time is a smaller percentage
of the total period. Unfortunately, the lower modulation fre-
quency has worse THD+N performance when the output
power is below 10 watts. The recommended switching fre-
quency to balance the THD+N performance, efficiency and
output power is 125kHz to 145kHz.
THD+N Measurements and Out of Audio Band Noise
THD+N (Total Harmonic Distortion plus Noise) is a very
important parameter by which all audio amplifiers are mea-
sured. Often it is shown as a graph where either the output
power or frequency is changed over the operating range. A
very important variable in the measurement of THD+N is the
bandwidth limiting filter at the input of the test equipment.
Class D amplifiers, by design, switch their output power
devices at a much higher frequency than the accepted audio
range (20Hz - 20kHz). Switching the outputs makes the
amplifier much more efficient than a traditional Class A/B
amplifier. Switching the outputs at high frequency also in-
creases the out-of-band noise. Under normal circumstances
this out-of-band noise is significantly reduced by the output
low pass filter. If the low pass filter is not optimized for a
given switching frequency, there can be significant increase
in out-of-band noise.
THD+N measurements can be significantly affected by
out-of-band noise, resulting in a higher than expected
THD+N measurement. To achieve a more accurate mea-
surement of THD, the bandwidth at the input of the test
equipment must be limited. Some common upper filter points
are 22kHz, 30kHz, and 80kHz. The input filter limits the
noise component of the THD+N measurement to a smaller
bandwidth resulting in a more real-world THD+N value.
The output low pass filter does not remove all of the switch-
ing fundamental and harmonics. If the switching frequency
fundamental is in the measurement range of the test equip-
ment, the THD+N measurement will include switching fre-
quency energy not removed by the output filter. Whereas the
switching frequency energy is not audible, it’s presence de-
grades the THD+N measurement. Reducing the bandwidth
to 30kHz and 22kHz reveals the true THD performance of
O
BYP
, and V
’s value to achieve the same filter cutoff frequency. The
BYP
O 2
is a capacitor connected between each output,
, and ground. This alternative is, however, not
1
) help filter the high frequency from the
BYP
.’
(Continued)
1
is 0.1µF to
BYP
.
13
the Class D amplifier. Increasing the switching frequency or
reducing the cutoff frequency of the output filter will also
reduce the level of the switching frequency fundamental and
it’s harmonics present at the output. This is caused by a
switching frequency that is higher than the output filter cutoff
frequency and, therefore, more attenuation of the switching
frequency.
In-band noise is higher in switching amplifiers than in linear
amplifiers because of increased noise from the switching
waveform. The majority of noise is out of band (as discussed
above), but there is also an increase of audible noise. The
output filter design (order and location of poles) has a large
effect on the audible noise level. Power supply voltage also
has an effect on noise level. The output filter removes a
certain amount of the switching noise. As the supply in-
creases, the attenuation by the output fiter is constant. How-
ever, the switching waveform is now much larger resulting in
higher noise levels.
THERMAL CONSIDERATIONS
Heat Sinking
The choice of a heat sink for the output FETs in a Class D
audio amplifier is made such that the die temperature does
not exceed T
cuitry under normal operating conditions. The heat sink
should be chosen to dissipate the maximum IC power which
occurs at maximum output power for a given load. Knowing
the maximum output power, the ambient temperature sur-
rounding the device, the load and the switching frequency,
the maximum power dissipation can be calculated. The ad-
ditional parameters needed are the maximum junction tem-
perature and the thermal resistance of the IC package (
junction to case), both of which are provided in the Absolute
Maximum Ratings and Operating Ratings sections above.
It should be noted that the idea behind dissipating the power
within the IC is to provide the device with a low resistance to
convection heat transfer such as a heat sink. Convection
cooling heat sinks are available commercially and their
manufacturers should be consulted for ratings. It is always
safer to be conservative in thermal design.
Proper IC mounting is required to minimize the thermal drop
between the package and the heat sink. The heat sink must
also have enough metal under the package to conduct heat
from the center of the package bottom to the fins without
excessive temperature drop. A thermal grease such as
Wakefield type 120 or Thermalloy Thermacote should be
used when mounting the package to the heat sink. Without
some thermal grease, the thermal resistance
sink) will be no better than 0.5˚C/W, and probably much
worse. With the thermal grease, the thermal resistance will
be 0.2˚C/W or less. It is important to properly torque the
mounting screw. Over tightening the mounting screw will
cause the package to warp and reduce the contact area with
the heat sink. It can also crack the die and cause failure of
the IC. The recommended maximum torque applied to the
mounting screw is 40 inch-lbs. or 3.3 foot-lbs.
Determining Maximum Power Dissipation
Power dissipation within the integrated circuit package is a
very important parameter. An incorrect maximum power dis-
sipation (P
ing, causing thermal shutdown circuitry to operate intermit-
tently. There are two components of power dissipation in a
class D amplifier. One component of power dissipation in the
D
) calculation may result in inadequate heat sink-
JMAX
and activate the thermal protection cir-
CS
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(case to
JC
,