Datasheet

LM4950
www.ti.com SNAS174E –JULY 2003–REVISED MAY 2013
POWER DISSIPATION
Power dissipation is a major concern when designing a successful single-ended or bridged amplifier. Equation 2
states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and
driving a specified output load.
P
DMAX-SE
= (V
DD
)
2
/ (2π
2
R
L
): Single Ended (2)
The LM4950's dissipation is twice the value given by Equation 2 when driving two SE loads. For a 12V supply
and two 8Ω SE loads, the LM4950's dissipation is 1.82W.
The LM4950's dissipation when driving a BTL load is given by Equation 3. For a 12V supply and a single 8Ω BTL
load, the dissipation is 3.65W.
P
DMAX-MONOBTL
= 4(V
DD
)
2
/ 2π
2
R
L
: Bridge Mode (3)
The maximum power dissipation point given by Equation 3 must not exceed the power dissipation given by
Equation 4:
P
DMAX
' = (T
JMAX
- T
A
) / θ
JA
(4)
The LM4950's T
JMAX
= 150°C. In the KTW package, the LM4950's θ
JA
is 20°C/W when the metal tab is soldered
to a copper plane of at least 16in
2
. This plane can be split between the top and bottom layers of a two-sided
PCB. Connect the two layers together under the tab with a 5x5 array of vias. For the NEC package, use an
external heatsink with a thermal impedance that is less than 20°C/W. At any given ambient temperature T
A
, use
Equation 4 to find the maximum internal power dissipation supported by the IC packaging. Rearranging
Equation 4 and substituting P
DMAX
for P
DMAX
' results in Equation 5. This equation gives the maximum ambient
temperature that still allows maximum stereo power dissipation without violating the LM4950's maximum junction
temperature.
T
A
= T
JMAX
- P
DMAX-MONOBTL
θ
JA
(5)
For a typical application with a 12V power supply and a BTL 8Ω load, the maximum ambient temperature that
allows maximum stereo power dissipation without exceeding the maximum junction temperature is approximately
77°C for the KTW package.
T
JMAX
= P
DMAX-MONOBTL
θ
JA
+ T
A
(6)
Equation 6 gives the maximum junction temperature T
JMAX
. If the result violates the LM4950's 150°C, reduce the
maximum junction temperature by reducing the power supply voltage or increasing the load resistance. Further
allowance 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 ambient temperatures are allowed as output
power or duty cycle decreases.
If the result of Equation 3 is greater than that of Equation 4, then decrease the supply voltage, increase the load
impedance, or reduce the ambient temperature. Further, ensure that speakers rated at a nominal 4Ω (SE
operation) or 8Ω (BTL operation) do not fall below 3Ω or 6Ω, respectively. If these measures are insufficient, a
heat sink can be added to reduce θ
JA
. The heat sink can be created using additional copper area around the
package, with connections to the ground pins, supply pin and amplifier output pins. Refer to the Typical
Performance Characteristics curves for power dissipation information at lower output power levels.
POWER SUPPLY VOLTAGE LIMITS
Continuous proper operation is ensured by never exceeding the voltage applied to any pin, with respect to
ground, as listed in Absolute Maximum Ratings section.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. Applications that employ a voltage regulator typically use a 10µF in parallel with a 0.1µF filter
capacitors to stabilize the regulator's output, reduce noise on the supply line, and improve the supply's transient
response. However, their presence does not eliminate the need for a local 1.0µF tantalum bypass capacitance
connected between the LM4950's supply pins and ground. Do not substitute a ceramic capacitor for the
tantalum. Doing so may cause oscillation. Keep the length of leads and traces that connect capacitors between
the LM4950's power supply pin and ground as short as possible. Connecting a 10µF capacitor, C
BYPASS
, between
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