Datasheet

TPA0211
2-W MONO AUDIO POWER AMPLIFIER
SLOS275D – JANUARY 2000 – REVISED NOVEMBER 2002
17
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
BTL amplifier efficiency (continued)
Table 2 employs equation 10 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.
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 (either SE or BTL) is how to manipulate the terms in the
efficiency equation to utmost advantage when possible. Note that in equation 10, 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.The TPA0211 data sheet shows that when
the TPA0211 is operating from a 5-V supply into a 4-Ω speaker 4-W peaks are available. Converting watts to
dB:
P
dB
+ 10Log
P
W
P
ref
+ 10Log
4W
1W
+ 6dB
(11)
Subtracting the headroom restriction to obtain the average listening level without distortion yields:
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)