Datasheet


SLOS386A − NOVEMBER 2001 − REVISED APRIL 2007
22
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APPLICATION INFORMATION
BTL amplifier efficiency (continued)
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 2.00 0.55
0.50 44.4 2.83 0.62
1.00 62.8 4.00 0.59
1.25 70.2 4.47
†
0.53
†
High peak voltages cause the THD to increase.
A final point to remember about Class-AB amplifiers is how to manipulate the terms in the efficiency equation
to utmost advantage when possible. Note that in equation 6, V
DD
is in the denominator. This indicates that as
V
DD
goes down, efficiency goes up.
crest factor and thermal considerations
Class-AB power amplifiers dissipate a significant amount of heat in the package under normal operating
conditions. A typical music CD requires 12 dB to 15 dB of dynamic range, or headroom above the average power
output, to pass the loudest portions of the signal without distortion. In other words, music typically has a crest
factor between 12 dB and 15 dB. When determining the optimal ambient operating temperature, the internal
dissipated power at the average output power level must be used. From the TPA6017A2 data sheet, one can
see that when the TPA6017A2 is operating from a 5-V supply into a 3-Ω speaker 4-W peaks are available.
Converting watts to dB:
P
dB
+ 10Log
P
W
P
ref
+ 10Log
4W
1W
+ 6dB
(7)
Subtracting the headroom restriction to obtain the average listening level without distortion yields:
6 dB − 18 dB = −12 dB (18 dB crest factor)
6 dB − 15 dB = −9 dB (15 dB crest factor)
6 dB − 12 dB = −6 dB (12 dB crest factor)
6 dB − 9 dB = −3 dB (9 dB crest factor)
6 dB − 6 dB = 0 dB (6 dB crest factor)
6 dB − 3 dB = 3 dB (3 dB crest factor)
Converting dB back into watts:
P
W
+ 10
PdBń10
P
ref
+ 63 mW (18 dB crest factor)
+ 125 mW (15 dB crest factor)
+ 250 mW (12 dB crest factor)
+ 500 mW (9 dB crest factor)
+ 1000 mW (6 dB crest factor)
(8
)
+ 2000 mW (3 dB crest factor)