LM4756 National Semiconductor Corporation, LM4756 Datasheet - Page 12

• Manufacturer Part Number: LM4756
• Description: Dual 7w Audio Power Amplifier W/mute, Standby And Volume Control
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
voltage swing will be reduced, creating a clipping effect as
shown in Figure 7. With further increases in die temperature
the maximum voltage swing will be further reduced.
The thermal sensing mechanism monitors the global die
temperature and is not intended to operate quickly enough to
shutdown the IC for extremely high power dissipation pulses
created by driving very low impedance loads.
In Figure 7, a 50kHz input signal is used to show the clipping
and attenuating effect of the LM4756 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
voltage variation and the highly reactive load impedance
variation 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 8Ω load can have an impedance dip down
to 5Ω at low frequencies. As well, the load is not purely
resistive, 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 increases.
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
required heat sink and are not intended to provide exact
power dissipation values.
FIGURE 7. Thermal Shutdown Response
P
DMAX
= 2(V
CCtot
2
/2π
2
R
L
)
(Continued)
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(2)
12
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
circuitry 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
minimum load impedance.
Referring to Figure 8, the thermal resistance from the die
(junction) to the outside air (ambient) is a combination of
three thermal resistances, θ
thermal resistances are provided by National, θ
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 LM4756 is equal to
the following:
The thermal resistance, θ
where θ
the case-to-sink thermal resistance (thermal compound),
and θ
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
θ
θ
(1) P
(2) θ
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 8Ω 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
resistances. Good washers can be obtained from Thermal-
loy or Berquist. Refer to the References list for contact
information for these manufacturers.
Supply Bypassing
The LM4756 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
JC
CS
CC
L
= 8Ω
θ
= 1˚C/W
= 0.5˚C/W
= +22V
SA
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.
= 2((22V)
is the junction-to-case thermal resistance, θ
JMAX
P
FIGURE 8. Thermal Model
DMAX
– T
2
/2π
AMB
= (T
2
(8Ω)) = 6W
) – P
JA
JMAX
JC
is equal to θ
, θ
DMAX
– T
CS
AMB
and θ
(θ
JC
)/θ
20064432
+ θ
JA
SA
JC
. Two of these
CS
+ θ
)]/P
JC
CS
and θ
DMAX
+ θ
CS
CS
SA
(3)
(4)
is
.
,