Datasheet

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SLOS327C − AUGUST 2000 − REVISED MAY 2001
19
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
BTL amplifier efficiency (continued)
(8
)
P
L
= Power delivered to load
P
SUP
= Power drawn from power supply
V
LRMS
= RMS voltage on BTL load
R
L
= Load resistance
V
P
= Peak voltage on BTL load
I
DD
avg = Average current drawn from the power supply
V
DD
= Power supply voltage
η
BTL
= Efficiency of a BTL amplifier
Therefore,
P
SUP
+
2V
DD
V
P
p R
L
substituting P
L
and P
SUP
into equation 7,
Efficiency of a BTL amplifier +
V
P
2
2R
L
2V
DD
V
P
p R
L
+
p V
P
4V
DD
V
P
+ 2P
L
R
L
Ǹ
h
BTL
+
p 2P
L
R
L
Ǹ
4V
DD
Where:
Therefore,
Table 2 employs equation 8 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 2. Efficiency vs Output Power in 5-V, 8-Ω BTL Systems
OUTPUT POWER
(W)
EFFICIENCY
(%)
PEAK VOLTAGE
(V)
INTERNAL DISSIPATION
(W)
0.25 31.4 2 0.55
0.5 44.4 2.83 0.62
1 62.8 4 0.59
1.25 70.2 4.47
†
0.53
†
High peak voltages cause the THD to increase.
A final point to remember about class-AB amplifiers (either SE or BTL) is how to manipulate the terms in the
efficiency equation to the utmost advantage when possible. Note that in equation 8, V
DD
is in the denominator.
This indicates that as V
DD
goes down, efficiency goes up.