Datasheet

LM4938
www.ti.com SNAS245B –FEBRUARY 2005–REVISED MAY 2013
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single
amplifier's output and ground. For a given supply voltage, bridge mode has a distinct advantage over the single-
ended configuration: its differential output doubles the voltage swing across the load. 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 or that the output signal is not clipped.
To ensure minimum output signal clipping when choosing an amplifier's closed-loop gain, refer to the AUDIO
POWER AMPLIFIER DESIGN section.
Another advantage of the differential bridge output is no net DC voltage across the load. This is accomplished by
biasing channel A's and channel B's outputs at half-supply. This eliminates the coupling capacitor that single
supply, single-ended amplifiers require. Eliminating an output coupling capacitor in a 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. 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
= (V
DD
)
2
/(2π
2
R
L
) Single-Ended (2)
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is higher
internal power dissipation for the same conditions.
The LM4938 has two operational amplifiers per channel. The maximum internal power dissipation per channel
operating in the bridge mode is four times that of a single-ended amplifier. From Equation 3), assuming a 5V
power supply and a 4Ω load, the maximum single channel power dissipation is 1.27W or 2.54W for stereo
operation.
P
DMAX
= 4 * (V
DD
)
2
/(2π
2
R
L
) Bridge Mode (3)
The LM4938's power dissipation is twice that given by Equation 2or Equation 3when operating in the single-
ended mode or bridge mode, respectively. Twice 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 LM4938's T
JMAX
= 150°C. In the MH package soldered to a DAP pad that expands to a copper area of 2in
2
on a PCB, the LM4938MH's θ
JA
is 41°C/W. At any given ambient temperature T
A
, use Equation 4to 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 LM4938's maximum junction temperature.
T
A
= T
JMAX
– 2*P
DMAX
θ
JA
(5)
For a typical application with a 5V power supply and an 4Ω load, the maximum ambient temperature that allows
maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 45°C
for the MH package.
T
JMAX
= P
DMAX
θ
JA
+ T
A
(6)
Equation 6 gives the maximum junction temperature T
JMAX
. If the result violates the LM4938's 150°C T
JMAX
,
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 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 2 is greater than that of Equation 3, 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 connections 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 Performance Characteristics curves for power
dissipation information at lower output power levels.
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