Datasheet

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USING LOW-ESR CAPACITORS
5-V VERSUS 3.3-V OPERATION
HEADROOM AND THERMAL CONSIDERATIONS
P
dB
 10Log
P
W
P
ref
 10Log
350 mW
1 W
 –4.6 dB
TPA321
SLOS312C – JUNE 2000 – REVISED JUNE 2004
Low-ESR capacitors are recommended throughout this application. A real (as opposed to ideal) capacitor can be
modeled simply as a resistor in series with an ideal capacitor. The voltage drop across this resistor minimizes the
beneficial effects of the capacitor in the circuit. The lower the equivalent value of this resistance, the more the
real capacitor behaves like an ideal capacitor.
The TPA321 operates over a supply range of 2.5 V to 5.5 V. This data sheet provides full specifications for 5-V
and 3.3-V operation, as these are considered to be the two most common standard voltages. There are no
special considerations for 3.3-V versus 5-V operation with respect to supply bypassing, gain setting, or stability.
The most important consideration is that of output power. Each amplifier in TPA321 can produce a maximum
voltage swing of V
DD
–1 V. This means, for 3.3-V operation, clipping starts to occur when V
O(PP)
= 2.3 V as
opposed to V
O(PP)
= 4 V at 5 V. The reduced voltage swing subsequently reduces maximum output power into an
8-Ω load before distortion becomes significant.
Operation from 3.3-V supplies, as can be shown from the efficiency formula in Equation 4 , consumes
approximately two-thirds the supply power for a given output-power level than operation from 5-V supplies.
Linear 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 headroom to pass the loudest portions without distortion
as compared with the average power output. The TPA321 data sheet shows that when the TPA321 is operating
from a 5-V supply into a 8-Ω speaker, 350 mW peaks are available. Converting watts to dB:
Subtracting the headroom restriction to obtain the average listening level without distortion yields:
4.6 dB – 15 dB = –19.6 dB (15-dB headroom)
4.6 dB – 12 dB = –16.6 dB (12-dB headroom)
4.6 dB – 9 dB = –13.6 dB (9-dB headroom)
4.6 dB – 6 dB = –10.6 dB (6-dB headroom)
4.6 dB – 3 dB = –7.6 dB (3-dB headroom)
Converting dB back into watts:
P
W
= 10
PdB/10
× P
ref
= 11 mW (15 dB headroom)
= 22 mW (12-dB headroom)
= 44 mW (9-dB headroom)
= 88 mW (6-dB headroom)
= 175 mW (3-dB headroom)
This is valuable information to consider when attempting to estimate the heat dissipation requirements for the
amplifier system. Comparing the absolute worst case, which is 350 mW of continuous power output with 0 dB of
headroom, against 12-dB and 15-dB applications drastically affects maximum ambient temperature ratings for
the system. Using the power dissipation curves for a 5-V, 8-Ω system, the internal dissipation in the TPA321 and
maximum ambient temperatures is shown in Table 2 .
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