LM4890_15 Datasheet, PDF (19/36 Pages) Texas Instruments – 1 Watt Audio Power Amplifier

LM4890
www.ti.com
SNAS138L – SEPTEMBER 2001 – REVISED MAY 2013
APPLICATION INFORMATION
BRIDGED CONFIGURATION EXPLANATION
As shown in Figure 7, the LM4890 has two operational amplifiers internally, allowing for a few different amplifier
configurations. 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_IN while the second amplifier's gain is fixed by the two internal 20kΩ resistors. Figure 7 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_IN)
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 the LM4890, 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.
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS FOR THE LM4890LD
The LM4890LD's exposed-DAP (die attach paddle) package (LD) provides a low thermal resistance between the
die and the PCB to which the part is mounted and soldered. The LM4890LD package should have its DAP
soldered to the grounded copper pad (heatsink) under the LM4890LD (the NC pins, no connect, and ground pins
should also be directly connected to this copper pad-heatsink area). The area of the copper pad (heatsink) can
be determined from the LD Power Derating graph. If the multiple layer copper heatsink areas are used, then
these inner layer or backside copper heatsink areas should be connected to each other with 4 (2 x 2) vias. The
diameter for these vias should be between 0.013 inches and 0.02 inches with a 0.050inch pitch-spacing. Ensure
efficient thermal conductivity by plating through and solder-filling the vias. Further detailed information concerning
PCB layout, fabrication, and mounting an WSON package is available from TI's Package Engineering Group
under application note AN1187.
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 LM4890 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/(2π²R_L)
(1)
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 additional copper foil, the
thermal resistance of the application can be reduced, resulting in higher P_DMAX. Additional copper foil can be
added to any of the leads connected to the LM4890. Refer to the Application Information on the LM4890
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.
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