LM4865
SNAS035G –DECEMBER 1999–REVISED MAY 2013
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APPLICATION INFORMATION
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 3, the LM4865 consists of two operational amplifiers internally. An external DC voltage sets
the closed-loop gain of the first amplifier, whereas two internal 20kΩresistors set the second amplifier's gain at -
1. The LM4865 can be used to drive a speaker connected between the two amplifier outputs or a monaural
headphone connected between VO1 and GND.
Figure 3 shows that the output of Amp1 serves as the input to Amp2. This results in both amplifiers producing
signals that are identical in magnitude, but 180° out of phase.
Taking advantage of this phase difference, a load placed between VO1 and VO2 is driven differentially (commonly
referred to as “bridge mode“ ). This mode is different from single-ended driven loads that are connected between
a single amplifier's output and ground.
Bridge mode has a distinct advantage over the single-ended configuration: its differential drive to the load
doubles the output swing for a specified supply voltage. This results in four times the output power when
compared to a single-ended amplifier under the same conditions. This increase in attainable output assumes that
the amplifier is not current limited or the output signal is not clipped.
Another advantage of the differential bridge output is no net DC voltage across load. This results from biasing
VO1 and VO2 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. The current flow created by the half-supply bias voltage 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 bridged or single-ended amplifier. Equation 1
states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and
driving a specified output load.
PDMAX = (VDD)2/(2π2RL) Single-Ended (1)
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is an increase
in internal power dissipation point for a bridge amplifier operating at the same given conditions.
PDMAX = 4*(VDD)2/(2π2RL) Bridge Mode (2)
The LM4865 has two operational amplifiers in one package and the maximum internal power dissipation is 4
times that of a single-ended amplifier. However, even with this substantial increase in power dissipation, the
LM4865 does not require heatsinking. From Equation 2, assuming a 5V power supply and an 8Ωload, the
maximum power dissipation point is 633 mW. The maximum power dissipation point obtained from Equation 2
must not be greater than the power dissipation that results from Equation 3:
PDMAX = (TJMAX–TA)/θJA (3)
For the micro SMD and SOIC packages, θJA = 150°C/W. The VSSOP package has a 190°C/W θJA. TJMAX =
150°C for the LM4865. For a given ambient temperature TA,Equation 3 can be used to find the maximum
internal power dissipation supported by the IC packaging. If the result of Equation 2 is greater than that of
Equation 3, then either decrease the supply voltage, increase the load impedance, or reduce the ambient
temperature. For a typical application using the micro SMD or SOIC packaged LM4865, a 5V power supply, and
an 8Ωload, the maximum ambient temperature that does not violate the maximum junction temperature is
approximately 55°C. The maximum ambient temperature for the VSSOP package with the same conditions is
approximately 30°C. These results further 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 decreases. Refer to the TYPICAL PERFORMANCE
CHARACTERISTICS curves for power dissipation information at lower output power levels.
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