lm4651 National Semiconductor Corporation, lm4651 Datasheet - Page 14

• 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
LM4652 is the R
current when operating at maximum output power. The other
component of power dissipation in the LM4652 is the switch-
ing loss. If the output power is high enough and the DC
resistance of the filter coils is not minimized then significant
loss can occur in the output filter. This will not affect the
power dissipation in the LM4652 but should be checked to
be sure that the filter coils with not over heat.
The first step in determining the maximum power dissipation
is finding the maximum output power with a given voltage
and load. Refer to the graph Output Power verses Supply
Voltage to determine the output power for the given load and
supply voltage. From this power, the RMS output current can
be calculated as I
dissipation caused by the output current is P
(I
found from the Electrical Characteristics for the LM4652
table above. The percentage of loss due to the switching is
calculated by Equation (9):
t
teristic for the LM4651 and Electrical Characteristic for
the LM4652 sections above. The system designer deter-
mines the value for f
pation caused by switching loss is found by Equation (10).
P
and the load impedance being used in the application. P
MAX
Supply Voltage in the Typical Performance Characteris-
tics section above.
P
nents (P
Determining the Correct Heat Sink
Once the LM4652’s power dissipation known, the maximum
thermal resistance (in ˚C/W) of a heat sink can be calculated.
This calculation is made using Equation (11) and is based on
the fact that thermal heat flow parameters are analogous to
electrical current flow properties.
Since we know
Maximum Ratings and Operating Ratings sections above
(taking care to use the correct
on which package type is being used in the application) and
have calculated P
the heat sink’s thermal resistance. The following equation is
derived from Equation (11):
r
, t
OUTRMS
OUTMAX
DMAX
f
P
and T
%LOSS
can be determined from the graph Output Power vs.
DMAX
for the LM4652 is found by adding the two compo-
SA
DSWITCH
P
)
is the 1% output power for the given supply voltage
2
OVERMOD
DSWITCH
= [(T
= (T
*
SWITCH
(1−%LOSS
(2 * R
JMAX
Where
JMAX
+ P
DMAX
DS (ON)
JC
OUTRMS
= (%LOSS
can be found in the Electrical Charac-
= (t
,
DS (ON)
SW
DOUT
− T
− T
and T
CS
r
of the FET times the RMS output
+ t
(switching frequency). Power dissi-
AMBIENTMAX
JA
SWITCH
AMBIENTMAX
) of power dissipation together.
, and T
). The value for R
f
=
= SQRT(P
+ T
AMBIENTMAX
SWITCH
JC
JC
OVERMOD
) (Watts) (10)
+
for the LM4652 depending
JMAX
) /
) / P
CS
*
OUT
P
from the Absolute
JA
+
, we only need
)
(Continued)
DMAX
OUTMAX
*
/R
SA
f
(Watts) (11)
SW
L
DS (ON)
). The power
] −
) /
JC
DOUT
can be
−
(9)
OUT -
SA
CS
=
,
14
Again, it must be noted that the value of
upon the system designer’s application and its correspond-
ing parameters as described previously. If the ambient tem-
perature surrounding the audio amplifier is higher than
T
given all other parameters are equal, will need to be lower.
Example Design of a Class D Amplifier
The following is an example of how to design a class D
amplifier system for a power subwoofer application utilizing
the LM4651 and LM4652 to meet the design requirements
listed below:
• Output Power, 1% THD
• Load Impedance
• Input Signal level
• Input Signal Bandwidth
• Ambient Temperature
Determine the Supply Voltage
From the graph Output Power verses Supply voltage at
1% THD the supply voltage needed for a 125 watt, 4
application is found to be
Determine the Value for R
The oscillation frequency is chosen to obtain a satisfactory
efficiency level while also maintaining a reasonable THD
performance. The modulation frequency can be chosen us-
ing the Clipping Power Point and Efficiency verses
Switching Frequency graph. A modulation frequency of
125kHz is found to be a good middle ground for THD per-
formance and efficiency. The value of the resistor for R
found from Equation (6) to be 3.9 k .
Determine the Value for R
The current limit is internally set as a failsafe to 10 amps.
The inductor ripple current and the peak output current must
be lower than 10 amps or current limit protection will turn on.
A typical 4 load driven by a filter using 50µH inductors does
not require more than 10A. The current limit will have to be
increased when loads less than 4
higher output power. With R
limit is 10A.
Determine the Value for R
The delay time or dead time is set to the recommended
value so R
is desired, R
value for R
GND.
Determine the Value of L
C
All component values show in Figure 1 Typical Audio Ap-
plication Circuit, are optimized for a subwoofer application.
Use the following guidelines when changing any component
values from those shown. The frequency response of the
output filter is controlled by L
plication Information section titled Output Stage Filtering
for a detailed explanation on calculating the correct values
for L
AMBIENTMAX
f
(the Output and Feedback Filters)
1
and C
DLY
DLY
BYP
DLY
, then the thermal resistance for the heat sink,
equals 5k . If a higher bandwidth of operation
is desired, connect the LM4651’s pin 17 to
.
should be a lower value resistor. If a zero
±
1
, C
SCKT
20V.
OSC
SCKT
DLY
1
BYP
(Modulation Frequency)
and C
(Dead Time Control)
equal to 100k , the current
(Circuit Limit)
, C
1
, R
BYP
are used to acheive
fl1
. Refer to the Ap-
R
SA
fl2
3V RMS (max)
10Hz − 150Hz
, C
is dependent
50˚C (max)
fl1
C
fl2
125W
OSC
, R
4
is
f
,