LM4753 National Semiconductor, LM4753 Datasheet - Page 11

• Manufacturer Part Number: LM4753
• Description: Dual 10W Audio Power Amplifier w/Mute/ Standby and Volume Control
• Manufacturer: National Semiconductor

Application Information
In Figure 7 , a 50 kHz input signal is used to show the clipping
and attenuating effect of the LM4753 when coming out of
thermal shutdown.
THERMAL CONSIDERATIONS
Determining Maximum Power Dissipation
It is important to determine the maximum amount of package
power dissipation in order to choose an adequate heat sink.
Improper heat sinking can lead to premature thermal shut-
down operation, causing music to cut out. Equation (2) can
be used to calculate the approximate maximum integrated
circuit power dissipation for your amplifier design, given the
supply voltage, and rated load, with both channels being
driven simultaneously.
To ensure that a proper heat sink is chosen, be sure to take
into account the effects of the unregulated power supply volt-
age variation and the highly reactive load impedance varia-
tion over frequency.
A poorly regulated power supply can have a supply voltage
variation of more than 10V. Be sure to take into account the
no-load power supply voltage.
A nominally rated 8 load can have an impedance dip down
to 5 at low frequencies. As well, the load is not purely resis-
tive, and this causes the amplifier output current to be out of
phase with the output voltage. When the current and voltage
are out of phase, the internal power dissipation actually in-
creases.
Equation (2) can be directly applied to the Power Dissipation
vs Output Power curves in the Typical Performance Charac-
teristics section. However, the curves take into account qui-
escent power dissipation which Equation (2) does not. The
curves are to be used as a guideline in determining the re-
quired heat sink and are not intended to provide exact power
dissipation values.
Heat Sinking
Choosing a heat sink for a high-power audio amplifier is
made entirely to keep the die temperature below its maxi-
mum junction temperature, so that the thermal protection cir-
cuitry does not operate under normal circumstances. The
heat sink should be chosen to dissipate the maximum IC
power for the maximum no-load supply voltage and the mini-
mum load impedance.
FIGURE 7. Thermal Shutdown Response
P
DMAX
= 2(V
CCtot
2
/2
2
R
(Continued)
L
)
DS100043-31
(2)
11
Referring to Figure 8 , the thermal resistance from the die
(junction) to the outside air (ambient) is a combination of
three thermal resistances,
thermal resistances are provided by National,
Since convection heat flow (power dissipation) is analogous
to current flow, thermal resistance is analogous to electrical
resistance, and temperature drops are analogous to voltage
drops, the power dissipation out of the LM4753 is equal to
the following:
The thermal resistance,
where
the case-to-sink thermal resistance (thermal compound),
and
Once the maximum power dissipation is calculated from
Equation (2) above, the minimum heat sink thermal resis-
tance can be calculated from Equation (4) below.
Example:
V
R
(1) P
(2)
19˚C/W
Therefore, the minimum heat sink thermal resistance re-
quired is 19˚C/W for both channels being driven simulta-
neously at maximum power dissipation into an 8 load using
a +22V voltage supply. Again, remember to take into account
the unregulated supply voltage and reactive load impedance
dips.
Should it be necessary to isolate the tab of the IC from the
heat sink, an insulating washer can be used. There are many
different types of insulating washers with varying thermal re-
sistances. Good washers can be obtained from Thermalloy
or Berquist. Refer to the References list for contact informa-
tion for these manufacturers.
Supply Bypassing
The LM4753 has good power supply rejection, however, for
all power amplifiers, proper power supply bypassing is re-
quired. To prevent oscillations and instability, all op amps
and power op amps should have their supply leads by-
passed with low-inductance capacitors having short leads.
All high frequency bypass capacitors should be located as
close to the package terminals as possible and have a clear
unobstructed current return path to ground. It is typical to use
capacitor values that are a factor of 100 different from each
other to minimize interaction with each other. The LM4753
should be bypassed with 0.1 µF ceramic and 100 µF tanta-
lum capacitors for optimum performance. The 100 µF tanta-
lum can be replaced with an electrolytic, but the bypassing
JC
CS
CC
L
= 8
= 1˚C/W
= 0.5˚C/W
SA
= +22V
DMAX
SA
SA
JC
= [(T
= [(150˚C–25˚C) – 6W(1˚C/W + 0.5˚C/W)]/6W =
is the sink-to-ambient thermal resistance.
is the junction-to-case thermal resistance,
= 2((22V)
JMAX
P
FIGURE 8. Thermal Model
DMAX
– T
2
/2
AMB
= (T
2
(8 )) = 6W
) – P
JA
JMAX
JC
is equal to
,
DMAX
– T
CS
AMB
and
(
DS100043-32
JC
)/
+
JA
SA
JC
. Two of these
CS
+
)]/P
JC
www.national.com
CS
and
DMAX
+
CS
CS
SA
(3)
(4)
is
.
,