LM4875_15 Datasheet, PDF (9/21 Pages) Texas Instruments – 750 mW Audio Power Amplifier with DC Volume Control and Headphone Switch

LM4875
www.ti.com
SNAS042C – JANUARY 2002 – REVISED MAY 2013
APPLICATION INFORMATION
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4875 consists of two operational amplifiers internally. An external DC voltage sets
the closed-loop gain of the first amplifier, whereas two internal 20kΩ resistors set the second amplifier's gain at -
1. The LM4875 can be used to drive a speaker connected between the two amplifier outputs or a monaural
headphone connected between V_O1 and GND.
Figure 1 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 V_O1 and V_O2 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
V_O1 and V_O2 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.
P_DMAX = (V_DD)²/(2π²R_L) 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.
P_DMAX = 4*(V_DD)²/(2π²R_L) Bridge Mode
(2)
The LM4875 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
LM4875 does not require heatsinking. From Equation 2, assuming a 5V power supply and an 8Ω load, 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:
P_DMAX = (T_JMAX–T_A)/θ_JA
(3)
For the SOIC package, θ_JA = 150°C/W. The VSSOP package has a 190°C/W θ_JA. T_JMAX = 150°C for the
LM4875. For a given ambient temperature T_A, 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 SOIC packaged LM4875, a 5V power supply, and an 8Ω load, 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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