Datasheet
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I
DD
RMS
2V
P
R
L
P
SUP
V
DD
I
DD
RMS
V
DD
2V
P
R
L
Efficiency
P
L
P
SUP
Where:
P
L
V
L
RMS
2
R
L
V
p
2
2R
L
V
L
RMS
V
P
2
(3)
Efficiency of a BTL configuration
V
P
2V
DD
P
L
R
L
2
12
2V
DD
(4)
TPA4861
SLOS163C – SEPTEMBER 1996 – REVISED JUNE 2004
APPLICATION INFORMATION (continued)
Table 1 employs Equation 4 to calculate efficiencies for four different output power levels. Note that the efficiency
of the amplifier is quite low for lower power levels and rises sharply as power to the load is increased, resulting in
a nearly flat internal power dissipation over the normal operating range. Note that the internal dissipation at full
output power is less than in the half power range. Calculating the efficiency for a specific system is the key to
proper power supply design. For a stereo 1-W audio system with 8-Ω loads and a 5-V supply, the maximum draw
on the power supply is almost 3.25 W.
Table 1. Efficiency Vs Output Power in 5-V 8-Ω BTL Systems
PEAK-TO-PEAK INTERNAL
OUTPUT POWER EFFICIENCY
VOLTAGE DISSIPATION
(W) (%)
(V) (W)
0.25 31.4 2.00 0.55
0.50 44.4 2.83 0.62
1.00 62.8 4.00 0.59
1.25 70.2 4.47
(1)
0.53
(1) High peak voltages cause the THD to increase.
A final point to remember about linear amplifiers, whether they are SE or BTL configured, is how to manipulate
the terms in the efficiency equation to utmost advantage when possible. Note that in Equation 4 , V
DD
is in the
denominator. This indicates that as V
DD
goes down, efficiency goes up.
For example, if the 5-V supply is replaced with a 10-V supply (TPA4861 has a maximum recommended V
DD
of
5.5 V) in the calculations of Table 1 , then efficiency at 1 W would fall to 31% and internal power dissipation
would rise to 2.18 W from 0.59 W at 5 V. Then for a stereo 1-W system from a 10-V supply, the maximum draw
would be almost 6.5 W. Choose the correct supply voltage and speaker impedance for the application.
15