LM4844 Datasheet, PDF (16/21 Pages) National Semiconductor (TI) – Stereo 1.2W Audio Sub-system with 3D Enhancement

Application Information (Continued)
This results in both amplifiers producing signals identical in
magnitude, but 180˚ out of phase. Taking advantage of this
phase difference, a load is placed between LLS- and LLS+
and driven differentially (commonly referred to as ’bridge
mode’). This results in a differential or BTL gain of:
A_VD = 2(R_f / R_i) = 2
(2)
Both the feedback resistor, R_f, and the input resistor, R_i, are
internally set.
Bridge mode amplifiers are different from single-ended am-
plifiers that drive loads connected between a single amplifi-
er’s output and ground. For a given supply voltage, bridge
mode has a distinct advantage over the single-ended con-
figuration: its differential output doubles the voltage swing
across the load. Theoretically, this produces four times the
output power when compared to a single-ended amplifier
under the same conditions. This increase in attainable output
power assumes that the amplifier is not current limited and
that the output signal is not clipped.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
LLS- and LLS+ outputs at half-supply. This eliminates the
coupling capacitor that single supply, single-ended amplifiers
require. Eliminating an output coupling capacitor in a typical
single-ended configuration forces a single-supply amplifier’s
half-supply bias voltage across the load. This increases
internal IC power dissipation and may permanently damage
loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier.
A direct consequence of the increased power delivered to
the load by a bridge amplifier is higher internal power dissi-
pation. The LM4844 has 2 sets of bridged-tied amplifier pairs
driving LLS and RLS. The maximum internal power dissipa-
tion operating in the bridge mode is twice that of a single-
ended amplifier. From Equation (3) and (4), assuming a 5V
power supply and an 8Ω load, the maximum power dissipa-
tion for LLS and RLS is 634mW per channel.
P_DMAX-LLS = 4(V_DD)² / (2π² R_L): Bridged
(3)
P_DMAX-RLS = 4(V_DD)² / (2π² R_L): Bridged
(4)
The LM4844 also has a pair of single-ended amplifiers driv-
ing LHP and RHP. The maximum internal power dissipation
for ROUT and LOUT is given by equation (5) and (6). From
Equations (5) and (6), assuming a 5V power supply and a
32Ω load, the maximum power dissipation for LOUT and
ROUT is 40mW per channel.
P_DMAX-LHP = (V_DD)² / (2π² R_L): Single-ended (5)
P_DMAX-RHP = (V_DD)² / (2π² R_L): Single-ended (6)
The maximum internal power dissipation of the LM4844
occurs during output modes 3, 8, and 13 when both loud-
speaker and headphone amplifiers are simultaneously on;
and is given by Equation (7).
P_DMAX-TOTAL =
P_DMAX-LLS + P_DMAX-RLS + P_DMAX-LHP + P_DMAX-RHP (7)
The maximum power dissipation point given by Equation (7)
must not exceed the power dissipation given by Equation
(8):
P_DMAX’ = (T_JMAX - T_A) / θ_JA
(8)
The LM4844’s T_JMAX = 150˚C. In the TL package, the
LM4844’s θ_JA is 62˚C/W. At any given ambient temperature
T_A, use Equation (8) to find the maximum internal power
dissipation supported by the IC packaging. Rearranging
Equation (8) and substituting P_DMAX-TOTAL for P_DMAX’ results
in Equation (9). This equation gives the maximum ambient
temperature that still allows maximum stereo power dissipa-
tion without violating the LM4844’s maximum junction tem-
perature.
T_A = T_JMAX - P_DMAX-TOTAL θ_JA
(9)
For a typical application with a 5V power supply, stereo 8Ω
loudspeaker load, and the stereo 32Ω headphone load, the
maximum ambient temperature that allows maximum stereo
power dissipation without exceeding the maximum junction
temperature is approximately 100˚C for the TL package.
T_JMAX = P_DMAX-TOTAL θ_JA + T_A
(10)
Equation (10) gives the maximum junction temperature T_J-
MAX. If the result violates the LM4844’s 150˚C, reduce the
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 a surface
mount part 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 (7) is
greater than that of Equation (8), then decrease the supply
voltage, increase the load impedance, or reduce the ambient
temperature. 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 con-
nections to the ground pin(s), supply pin and amplifier output
pins. External, solder attached SMT heatsinks such as the
Thermalloy 7106D can also improve power dissipation.
When adding a heat sink, the θ_JA is the sum of θ_JC, θ_CS, and
θ_SA. (θ_JC is the junction-to-case thermal impedance, θ_CS is
the case-to-sink thermal impedance, and θ_SA is the sink-to-
ambient thermal impedance.) Refer to the Typical Perfor-
mance Characteristics curves for power dissipation informa-
tion at lower output power levels.
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