LM4752T National Semiconductor, LM4752T Datasheet - Page 12

• Manufacturer Part Number: LM4752T
• Description: Stereo 11W Audio Power Amplifier
• Manufacturer: National Semiconductor

www.national.com
Application Information
put signal. An external circuit may be used to sense for the
desired threshold, and pull the bias line (pin5) to ground to
disable the input preamp. Figure 7 shows an example of
such a circuit. When the voltage across the zener diode
drops below its threshold, current flow into the base of Q1 is
interrupted. Q2 then turns on, discharging the bias capacitor.
This discharge rate is governed by several factors, including
the bias capacitor value, the bias voltage, and the resistor at
the emitter of Q2. An equation for approximating the value of
the emitter discharge resistor, R, is given below:
Note that this is only a linearized approximation based on a
discharge time of 0.1s. The circuit should be evaluated and
adjusted for each application.
As mentioned earlier in the Application Circuit with Mute
section, when using an external circuit to pull down the bias
line, the rate of discharge will have an effect on the turn-off
induced distortions. Please refer to the Application Circuit
with Mute section for more information.
THERMAL CONSIDERATIONS
Heat Sinking
Proper heatsinking is necessary to ensure that the amplifier
will function correctly under all operating conditions. A heat-
sink that is too small will cause the die to heat excessively
and will result in a degraded output signal as the internal
thermal protection circuitry begins to operate.
The choice of a heatsink for a given application is dictated by
several factors: the maximum power the IC needs to dissi-
pate, the worst-case ambient temperature of the circuit, the
junction-to-case thermal resistance, and the maximum junc-
tion temperature of the IC. The heat flow approximation
equation used in determining the correct heatsink maximum
thermal resistance is given below:
where:
P
T
T
0.2 to 0.5 ˚C/W)
JC
CS
SA
J
A
DMAX
(˚C) = junction temperature of the IC
(˚C) = ambient temperature
(˚C/W) = junction-to-case thermal resistance of the IC
(˚C/W) = case-to-heatsink thermal resistance (typically
(˚C/W) = thermal resistance of heatsink
FIGURE 7. External Undervoltage Pull-Down
= maximum power dissipation of the IC
T
R = (0.7V) / (C
J
–T
A
= P
DMAX
B
• (
• (V
JC
S
+
/ 2) / 0.1s)
CS
(Continued)
+
DS100039-32
SA
)
12
When determining the proper heatsink, the above equation
should be re-written as:
TO-263 HEATSINKING
Surface mount applications will be limited by the thermal dis-
sipation properties of printed circuit board area. The TO-263
package is not recommended for surface mount applications
with V
There are TO-263 package enhancements, such as clip-on
heatsinks and heatsinks with adhesives, that can be used to
improve performance.
Standard FR-4 single-sided copper clad will have an ap-
proximate Thermal resistance (
The above values for
proportions (i.e. variations in width and length will vary
For audio applications, where peak power levels are short in
duration, this part will perform satisfactory with less
heatsinking/copper clad area. As with any high power design
proper bench testing should be undertaken to assure the de-
sign can dissipate the required power. Proper bench testing
requires attention to worst case ambient temperature and air
flow. At high power dissipation levels the part will show a ten-
dency to increase saturation voltages, thus limiting the un-
distorted power levels.
Determining Maximum Power Dissipation
For a single-ended class AB power amplifier, the theoretical
maximum power dissipation point is a function of the supply
voltage, V
following equation:
(single channel)
The above equation is for a single channel class-AB power
amplifier. For dual amplifiers such as the LM4752, the equa-
tion for calculating the total maximum power dissipated is:
(dual channel)
or
(Bridged Outputs)
P
Heatsink Design Example:
Determine the system parameters:
Device parameters from the datasheet:
Calculations:
Conclusion: Choose a heatsink with
1.5 x 1.5 in. sq.
2 x 2 in. sq.
V
R
T
T
2 • P
DMAX
SA
JC
A
L
J
S
= 4
= 55˚C
= 150˚C
= 24V
= 2˚C/W
DMAX
S
[ (T
(W) = 4[V
>
/ 14.6W ] − 2˚C/W − 0.2˚C/W = 4.3˚C/W
J
P
S
SA
16V due to limited printed circuit board area.
, and the load resistance, R
− T
DMAX
= 2 • [V
P
DMAX
A
) / P
[ (T
S
(W) = 2 • [V
2
Operating Supply Voltage
Minimum load impedance
Worst case ambient temperature
Maximum junction temperature
Junction-to-case thermal resistance
(W) = [V
J
/ (2
S
DMAX
20–27˚C/W
16–23˚C/W
− T
2
V
4 ) = 14.6W
/ (2 •
S
SA
2
A
2
] −
) / P
/ (
• R
vary widely due to dimensional
S
L
2
DMAX
2
)]
2
S
JC
/ (2 •
2
SA
• R
• R
/ (2 •
−
) ranging from:
] −
L
L
(T
testing, 1 oz. Copper)
)
) ] = (24V)
CS
SA
A
2
L
=28˚C, Sine wave
= [ (150˚C − 55˚C)
JC
• R
and is given by the
2
• R
4.3˚C/W.
−
L
) ]
L
CS
) ]
2
/ (2 •
SA
2
•
).