Datasheet
SLOS230D − NOVEMBER 1998 − REVISED OCTOBER 2002
22
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
BTL amplifier efficiency (continued)
Although the voltages and currents for SE and BTL are sinusoidal in the load, currents from the supply are very
different between SE and BTL configurations. In an SE application the current waveform is a half-wave rectified
shape, whereas in BTL it is a full-wave rectified waveform. This means RMS conversion factors are different.
Keep in mind that for most of the waveform, both the push and pull transistors are not on at the same time, which
supports the fact that each amplifier in the BTL device only draws current from the supply for half the waveform.
The following equations are the basis for calculating amplifier efficiency.
I
DD
rms +
2V
P
p R
L
P
SUP
+ V
DD
I
DD
rms +
V
DD
2V
P
p R
L
Efficiency +
P
L
P
SUP
Efficiency of a BTL configuration +
p V
P
4V
DD
+
p
ǒ
2P
L
R
L
Ǔ
1ń2
4V
DD
(3
)
where
(4
)
P
L
+
V
L
rms
2
R
L
+
V
p
2
2R
L
V
L
rms +
V
P
2
Ǹ
Table 1 employs equation 4 to calculate efficiencies for three different output power levels. 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. 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.
Table 1. Efficiency Vs Output Power in 3.3-V 8-Ω BTL Systems
OUTPUT POWER
(W)
EFFICIENCY
(%)
PEAK VOLTAGE
(V)
INTERNAL
DISSIPATION
(W)
0.125 33.6 1.41 0.26
0.25 47.6 2.00 0.29
0.375 58.3 2.45
†
0.28
†
High-peak voltage values cause the THD to increase.
A final point to remember about linear amplifiers (either SE or BTL) is how to manipulate the terms in the
efficiency equation to utmost advantage when possible. In equation 4, V
DD
is in the denominator. This indicates
that as V
DD
goes down, efficiency goes up.