Datasheet

LM4819, LM4819MBD
SNAS133D –FEBRUARY 2001–REVISED MARCH 2013 www.ti.com
APPLICATION INFORMATION
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4819 consists of two operational amplifiers. External resistors, R
i
and R
F
set the
closed-loop gain of the first amplifier (and the amplifier overall), whereas two internal 20kΩ resistors set the
second amplifier's gain at -1. The LM4819 is typically used to drive a speaker connected between the two
amplifier outputs.
Figure 1 shows that the output of Amp1 servers as the input to Amp2, which results in both amplifiers producing
signals identical in magnitude but 180° out of phase. Taking advantage of this phase difference, a load is placed
between V
01
and V
02
and driven differentially (commonly referred to as "bridge mode"). This results in a
differential gain of
A
VD
= 2 *(R
f
/R
i
) (1)
Bridge mode is 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 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. To ensure minimum
output signal clipping when choosing an amplifier's closed-loop gain, refer to the Audio Power Amplifier Design
Example section.
Another advantage of the differential bridge output is no net DC voltage across the load. This results from biasing
V
01
and V
02
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 2
states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and
driving a specified load.
P
DMAX
= (V
DD
)
2
/(2π
2
R
L
) (W) Single-ended (2)
However, a direct consequence of the increased power delivered to the load by a bridged amplifier is an increase
in the internal power dissipation point for a bridge amplifier operating at the same given conditions. Equation 3
states the maximum power dissipation point for a bridged amplifier operating at a given supply voltage and
driving a specified load.
P
DMAX
= 4(V
DD
)
2
/(2π
2
R
L
) (W) Bridge Mode (3)
The LM4819 has two operational amplifiers in one package and the maximum internal power dissipation is four
times that of a single-ended amplifier. However, even with this substantial increase in power dissipation, the
Lm4819 does not require heatsinking. From Equation 3, assuming a 5V power supply and an 8Ω load, the
maximum power dissipation point is 633mW. The maximum power dissipation point obtained from Equation 3
must not exceed the power dissipation predicted by Equation 4:
P
DMAX
= (T
JMAX
- T
A
)/θ
JA
(W) (4)
For the micro DGK0008A package, θ
JA
= 210°C/W, for the D0008A package, θ
JA
= 170°C/W , and T
JMAX
= 150°C
for the LM4819. For a given ambient temperature, T
A
, Equation 4 can be used to find the maximum internal
power dissipation supported by the IC packaging. If the result of Equation 3 is greater than the result of
Equation 4, then decrease the supply voltage, increase the load impedance, or reduce the ambient temperature.
For a typical application using the D0008A packaged LM4819 with a 5V power supply and an 8Ω load, the
maximum ambient temperature that does not violate the maximum junction temperature is approximately 42°C. If
a DGK0008A packaged part is used instead with the same supply voltage and load, the maximum ambient
temperature is 17°C. In both cases, it is assumed that a device is a surface mount part operating around the
maximum power dissipation point. The assumption that the device is operating around the maximum power
dissipation point is incorrect for an 8Ω load. The maximum power dissipation point occurs when the output power
is equal to the maximum power dissipation or 50% efficiency. The LM4819 is not capable of the output power
level (633mW) required to operate at the maximum power dissipation point for an 8Ω load. To find the maximum
power dissipation, the graph Figure 22 must be used. From the graph, the maximum power dissipation for an 8Ω
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