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10

C

B

250 k

Ω

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 3500 mW + –4.6 dB

TPA301

SLOS208E – JANUARY 1998 – REVISED JUNE 2004

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, appearing 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, which ensures the input capacitor is fully charged

before the bypass capacitor is fully charged and the amplifier starts up.

values into the Equation 10 we get: 18.2

≤ 35.5, which satisfies the rule. Recommended values for bypass

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 TPA301 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 TPA301 can produce a maximum

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. From the TPA301 data sheet, one can see that when the TPA301

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)

18