TPA731
700mW MONO LOW VOLTAGE AUDIO POWER AMPLIFIER
WITH DIFFERENTIAL INPUTS
SLOS315B JUNE 2000 REVISED OCTOBER 2002
18
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
APPLICATION INFORMATION
midrail bypass capacitor, CB
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
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.
(10)
10
C
B
250 k
v
1
R
F )
R
I
C
I
As an example, consider a circuit where CB is 2.2 F, CI is 0.47 F, RF is 50 k, and RI is 10 k. Inserting these
values into the equation 10 we get:
18.2
v 35.5
which satisfies the rule. Bypass capacitor, CB, values of 0.1 F to 2.2 F ceramic or tantalum low-ESR capacitors
are recommended for the best THD and noise performance.
using low-ESR capacitors
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.
5-V versus 3.3-V operation
The TPA731 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 TPA731 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 at 5 V. The reduced voltage swing subsequently reduces maximum output power into
an 8-
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.