lm4651 National Semiconductor Corporation, lm4651 Datasheet - Page 12

• 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
Introduction
National Semiconductor (NSC) is committed to providing
application information that assists our customers in obtain-
ing the best performance possible from our products. The
following information is provided in order to support this
commitment. The reader should be aware that the optimiza-
tion of performance was done using a reference PCB de-
signed by NSC and shown in Figure 7 through 11. Variations
in performance can occur because of physical changes in
the printed circuit board and the application. Therefore, the
designer should know that component value changes may
be required in order to optimize performance in a given
application. The values shown in this data sheet can be used
as a starting point for evaluation purposes. When working
with high frequency circuits, good layout practices are also
critical to achieving maximum performance.
Input Pre-Amplifier with Subwoofer Filter
The LM4651 and LM4652 Class D solution is designed for
low frequency audio applications where low gain is required.
This necessitates a pre−amplifier stage with gain and a low
pass audio filter. An inexpensive input stage can be de-
signed using National’s LM833 audio operational amplifier
and a minimum number of external components. A gain of 10
(20dB) is recommended for the pre−amplifier stage. For a
subwoofer application, the pole of the low pass filter is
normally set within the range of 60Hz − 180Hz. For a clean
sounding subwoofer the filter should be at least a
second-order filter to sharply roll off the high frequency audio
signals. A higher order filter is recommended for stand-alone
self-powered subwoofer applications. Figure 6 shows a
simple input stage with a gain of 10 and a second-order low
pass filter.
Supply Bypassing
Correct supply bypassing has two important goals. The first
is to ensure that noise on the supply lines does not enter the
circuit and become audible in the output. The second is to
help stabilize an unregulated power supply and provide cur-
rent under heavy current conditions. Because of the two
different goals multiple capacitors of various types and val-
ues are recommended for supply bypassing. For noise
de-coupling, generally small ceramic capacitors (.001µF to
.1µF) along with slightly larger tantalum or electrolytic ca-
pacitors (1µF to 10µF) in parallel will do an adequate job of
removing most noise from the supply rails. These capacitors
should be placed as close as possible to each IC’s supply
FIGURE 6. Pre−amplifier Stage with Low Pass Filter
(Continued)
DS101277-77
12
pin(s) using leads as short as possible. For supply stabiliz-
ing, large electrolytic capacitors (3,300µF to 15,000µF) are
needed. The value used is design and cost dependent.
High Frequency PCB Design
A double-sided PCB is recommended when designing a
class D amplifier system. One side should contain a ground
plane with the power traces on the other side directly over
the ground plane. The advantage is the parasitic capaci-
tance created between the ground plane and the power
planes. This parasitic capacitance is very small (pF) but is
the value needed for coupling high frequency noise to
ground. At high frequencies, capacitors begin to act more
like inductors because of lead and parasitic inductance in the
capacitor. For this reason, bypassing capacitors should be
surface mount because of their low parasitic inductance.
Equation (8) shows how to determine the amount of power to
ground plane capacitance.
A is the common PCB area and d is the distance between
the planes. The designer should target a value of 100pF or
greater for both the positive supply to ground capacitance
and negative supply to ground capacitance. Signal traces
that cross over each other should be laid out at 90˚ to
minimized any coupling.
Output Offset Voltage Minimization
The amount of DC offset voltage seen at the output with no
input signal present is already quite good with the LM4651/
52. With no input signal present the system should be at
50% duty cycle. Any deviation from 50% duty cycle creates a
DC offset voltage seen by the load. To completely eliminate
the DC offset, a DC voltage divider can be used at the input
to set the DC offset to near zero. This is accomplished by a
simple resistor divider that applies a small DC voltage to the
input. This forces the duty cycle to 50% when there is no
input signal. The result is a LM4651 and LM4652 system
with near zero DC offset. The divider should be a 1.8M
from the +6V output (pin 6) to the input (other side of 25k,
R
a 1µF input capacitor before R
the source. R
with a 3dB point at 6.35Hz. The value of R
according to the application. Variations in switching fre-
quency and supply voltage will change the amount of offset
voltage requiring a different value than stated above. The
value above (1.8M ) is for
of 125kHz.
Output Stage Filtering
As common with Class D amplifier design, there are many
trade-offs associated with different circuit values. The output
stage is not an exception. National has found good results
with a 50µF inductor and a 5µF Mylar capacitor (see Figure
1, Typical Audio Application Circuit) used as the output
LC filter. The two-pole filter contains three components; L
and C
output. The design formula for a bridge output filter is f
1/[2 (L
A common mistake is to connect a large capacitor between
ground and each output. This applies only to single-ended
1
). R
BYP
1
1
acts like the second resistor in the divider. Also use
2C
because the LM4651 and LM4652 have a bridged
BYP
1
where eo = 0.22479pF/in and er = 4.1
)
and the 1µF capacitor create a high pass filter
C = eoerA/d
1
⁄
2
].
±
20V and a switching frequency
1
to block the DC voltage from
(Farads)
OFFSET
(8)
is set
C
=
1