lm4940tsx National Semiconductor Corporation, lm4940tsx Datasheet - Page 9

• Manufacturer Part Number: lm4940tsx
• Description: 6w Streo Audio Power Amplifier
• Manufacturer: National Semiconductor Corporation

Application Information
HIGH VOLTAGE BOOMER WITH INCREASED OUTPUT
POWER
Unlike previous 5V Boomer
signed to operate over a power supply voltages range of 10V
to 15V. Operating on a 12V power supply, the LM4940 will
deliver 3.1W per channel into 4Ω loads with no more than
1% THD+N.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended amplifier. Equation (2) states the
maximum power dissipation point for a single-ended ampli-
fier operating at a given supply voltage and driving a speci-
fied output load.
The LM4940’s dissipation is twice the value given by Equa-
tion (2) when driving two SE loads. For a 12V supply and two
8Ω SE loads, the LM4940’s dissipation is 1.82W.
The maximum power dissipation point (twice the value given
by Equation (2)) must not exceed the power dissipation
given by Equation (4):
The LM4940’s T
LM4940’s θ
copper plane of at least 16in
tween the top and bottom layers of a two-sided PCB. Con-
P
DMAX-SE
JA
is 20˚C/W when the metal tab is soldered to a
P
= (V
JMAX
DMAX
DD
’ = (T
= 150˚C. In the TS package, the
)
2
/ (2π
®
JMAX
2
FIGURE 3. Typical LM4940 Stereo Amplifier Application Circuit
amplifiers, the LM4940 is de-
. This plane can be split be-
2
R
- T
L
): Single Ended
A
) / θ
JA
(1)
(2)
9
nect the two layers together under the tab with a 5x5 array of
vias. For the TA package, use an external heatsink with a
thermal impedance that is less than 20˚C/W. At any given
ambient temperature T
mum internal power dissipation supported by the IC packag-
ing. Rearranging Equation (4) and substituting P
P
mum ambient temperature that still allows maximum stereo
power dissipation without violating the LM4940’s maximum
junction temperature.
For a typical application with a 12V power supply and two 4Ω
SE loads, the maximum ambient temperature that allows
maximum stereo power dissipation without exceeding the
maximum junction temperature is approximately 113˚C for
the TS package.
Equation (6) gives the maximum junction temperature
T
maximum junction temperature by reducing the power sup-
ply voltage or increasing the load resistance. Further allow-
ance should be made for increased ambient temperatures.
The above examples assume that a device is operating
around the maximum power dissipation point. Since internal
power dissipation is a function of output power, higher am-
bient temperatures are allowed as output power or duty
cycle decreases.
JMAX
DMAX
. If the result violates the LM4940’s 150˚C, reduce the
’ results in Equation (5). This equation gives the maxi-
T
T
JMAX
A
= T
A
= P
JMAX
, use Equation (4) to find the maxi-
DMAX-SE
- P
DMAX-SE
θ
JA
+ T
20075672
θ
JA
A
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DMAX
(3)
(4)
for