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TPA721 Datasheet(PDF) 17 Page - Texas Instruments

Part No. TPA721
Description  700-mW MONO LOW-VOLTAGE AUDIO POWER AMPLIFIER
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

TPA721 Datasheet(HTML) 17 Page - Texas Instruments

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Midrail Bypass Capacitor, CB
10
C
B
250 k
v
1
R
F )
R
I
C
I
(10)
USING LOW-ESR CAPACITORS
5-V VERSUS 3.3-V OPERATION
HEADROOM AND THERMAL CONSIDERATIONS
P
dB +
10LogP
W +
10Log 700 mW + –1.5 dB
TPA721
SLOS231E – NOVEMBER 1998 – REVISED JUNE 2004
The midrail bypass capacitor, CB, is the most critical capacitor and serves several important functions. During
start-up or recovery from shutdown mode, CB determines the rate at which the amplifier starts up. The second
function is to reduce noise produced by the power supply caused by coupling into the output drive signal. This
noise is from the midrail generation circuit internal to the amplifier, which appears as degraded PSRR and THD +
N. The capacitor is fed from a 250-k
Ω source inside the amplifier. To keep the start-up pop as low as possible,
the relationship shown in Equation 10 should be maintained. This insures the input capacitor is fully charged
before the bypass capacitor is fully charged and the amplifier starts up.
As an example, consider a circuit where CB is 2.2 µF, CI is 0.47 µF, RF is 50 kΩ, and RI is 10 kΩ. Inserting these
values into the Equation 10 results in:
18.2
≤ 35.5
which satisfies the rule. Recommended value for bypass capacitor CB is 0.1-µF to 2.2-µF, ceramic or tantalum
low-ESR, for the best THD and noise performance.
Low-ESR capacitors are recommended throughout this applications section. 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 TPA721 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 TPA721 can produce a maximum
voltage swing of VDD –1 V. This means, for 3.3-V operation, clipping starts to occur when VO(PP) = 2.3 V as
opposed to VO(PP) = 4 V for 5-V operation. 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 of operation from 5-V supplies for a given output-power level.
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 TPA721 data sheet shows that when the TPA721 is operating
from a 5-V supply into an 8-
Ω speaker, 700 mW peaks are available. Converting watts to dB:
Subtracting the headroom restriction to obtain the average listening level without distortion yields:
–1.5 dB – 15 dB = –16.5 (15-dB headroom)
–1.5 dB – 12 dB= –13.5 (12-dB headroom)
–1.5 dB – 9 dB = –10.5 (9-dB headroom)
–1.5 dB – 6 dB = –7.5 (6-dB headroom)
–1.5 dB – 3 dB= –4.5 (3-dB headroom
17


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