Datasheet

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CREST FACTOR AND THERMAL CONSIDERATIONS
P = 10Log
dB
= 10Log
P
W
P
ref
3 W
1 W
= 5 dB
(9)
P = 10 x P
W ref
PdB/10
(10)
TPA6021A4
SLOS465 – JUNE 2005
Table 3. Efficiency vs Output Power in 5-V, 8- Ω BTL Systems
OUTPUT POWER EFFICIENCY PEAK VOLTAGE INTERNAL 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 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 8, V
DD
is in the denominator. This
indicates that as V
DD
goes down, efficiency goes up.
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 data sheet graph (Figure 5.), one
can see that when the TPA6021A4 is operating from a 5-V supply into a 4- Ω speaker at 1% THD, that output
power is 1.5-W so maximum instantaneous output power is 3-W. Use equation 9 to convert watts to dB.
Subtracting the headroom restriction to obtain the average listening level without distortion yields:
5 dB - 15 dB = –10 dB (15-dB crest factor)
5 dB - 12 dB = –7 dB (12-dB crest factor)
5 dB - 9 dB = –4 dB (9-dB crest factor)
5 dB - 6 dB = -1 dB (6-dB crest factor)
5 dB - 3 dB = 2 dB (3-dB crest factor)
To convert dB back into watts use equation 10.
= 48 mW (18-dB crest factor)
= 95 mW (15-dB crest factor)
= 190 mW (12-dB crest factor)
= 380 mW (9-dB crest factor)
= 750 mW (6-dB crest factor)
= 1500 mW (3-dB crest factor)
This is valuable information to consider when attempting to estimate the heat dissipation requirements for the
amplifier system. Comparing the worst case, which is 1.5 W of continuous power output with a 3-dB crest factor,
against 12-dB and 15-dB applications significantly affects maximum ambient temperature ratings for the system.
Using the power dissipation curves for a 5-V, 4- Ω system, the internal dissipation in the TPA6021A4 and
maximum ambient temperatures is shown in Table 4 .
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