Datasheet

LM4995, LM4995TMBD
SNAS329G –APRIL 2006–REVISED APRIL 2013 www.ti.com
APPLICATION INFORMATION
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4995 has two internal operational amplifiers. The first amplifier's gain is externally
configurable, while the second amplifier is internally fixed in a unity-gain, inverting configuration. The closed-loop
gain of the first amplifier is set by selecting the ratio of R
f
to R
i
while the second amplifier's gain is fixed by the
two internal 20kΩ resistors. Figure 1 shows that the output of amplifier one serves as the input to amplifier two
which results in both amplifiers producing signals identical in magnitude, but out of phase by 180°. Consequently,
the differential gain for the IC is
A
VD
= 2 *(R
f
/R
i
) (1)
By driving the load differentially through outputs Vo1 and Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is different from the classical single-ended amplifier
configuration where one side of the load is connected to ground.
A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides
differential drive to the load, thus doubling output swing for a specified supply voltage. Four times the output
power is possible as 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 clipped. In order to choose an amplifier's closed-
loop gain without causing excessive clipping, please refer to the AUDIO POWER AMPLIFIER DESIGN section.
A bridge configuration, such as the one used in LM4995, also creates a second advantage over single-ended
amplifiers. Since the differential outputs, Vo1 and Vo2, are biased at half-supply, no net DC voltage exists across
the load. This eliminates the need for an output coupling capacitor which is required in a single supply, single-
ended amplifier configuration. Without an output coupling capacitor, the half-supply bias across the load would
result in both increased internal IC power dissipation and also possible loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an
increase in internal power dissipation. Since the LM4995 has two operational amplifiers in one package, the
maximum internal power dissipation is 4 times that of a single-ended amplifier. The maximum power dissipation
for a given application can be derived from the power dissipation graphs or from Equation (1).
P
DMAX
= 4*(V
DD
)
2
/(2π
2
R
L
) (2)
It is critical that the maximum junction temperature T
JMAX
of 150°C is not exceeded. T
JMAX
can be determined
from the power derating curves by using P
DMAX
and the PC board foil area. By adding copper foil, the thermal
resistance of the application can be reduced from the free air value of θ
JA
, resulting in higher P
DMAX
values
without thermal shutdown protection circuitry being activated. Additional copper foil can be added to any of the
leads connected to the LM4995. It is especially effective when connected to V
DD
, GND, and the output pins.
Refer to the application information on the LM4995 reference design board for an example of good heat sinking.
If T
JMAX
still exceeds 150°C, then additional changes must be made. These changes can include reduced supply
voltage, higher load impedance, or reduced ambient temperature. Internal power dissipation is a function of
output power. Refer to the TYPICAL PERFORMANCE CHARACTERISTICS curves for power dissipation
information for different output powers and output loading.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for low noise performance and high supply rejection.
The capacitor location on both the bypass and power supply pins should be as close to the device as possible. A
ceramic 0.1μF placed in parallel with the tantalum 2.2μF bypass (C
B
) capacitor will aid in supply stability. This
does not eliminate the need for bypassing the power supply pins of the LM4995. The selection of a bypass
capacitor, especially C
B
, is dependent upon PSRR requirements, click and pop performance (as explained in the
section, PROPER SELECTION OF EXTERNAL COMPONENTS), system cost, and size constraints.
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