C
i +
1
2p Z
i fc
(6)
Power Supply Decoupling, CS
Midrail Bypass Capacitor, CBYP
Differential Input
Shutdown Modes
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The value of CI is important, as it directly affects the bass (low frequency) performance of the circuit. Consider
the example where ZI is 20 k and the specification calls for a flat bass response down to 80 Hz. Equation 5 is In this example, Ci is 0.1 F, so one would likely choose a value in the range of 0.1 F to 1 F. If the gain is
known and will be constant, use Zi from Table 1 to calculate CI. A further consideration for this capacitor is the
leakage path from the input source through the input network (Ci) and the feedback network to the load. This
leakage current creates a dc offset voltage at the input to the amplifier that reduces useful headroom, especially
in high-gain applications. For this reason a low-leakage tantalum or ceramic capacitor is the best choice. When
polarized capacitors are used, the positive side of the capacitor should face the amplifier input in most
applications as the dc level there is held at VDD/2, which is likely higher than the source dc level. It is important to
confirm the capacitor polarity in the application.
The TPA2000D1 is a high-performance CMOS audio amplifier that requires adequate power supply decoupling
to ensure that the output total harmonic distortion (THD) is as low as possible. Power supply decoupling also
prevents oscillations for long lead lengths between the amplifier and the speaker. The optimum decoupling is
achieved by using two capacitors of different types that target different types of noise on the power supply leads.
For higher frequency transients, spikes, or digital hash on the line, a good low equivalent-series-resistance (ESR)
ceramic capacitor, typically 0.1
F, placed as close as possible to the device V
DD lead works best. For filtering
lower-frequency noise signals, a larger aluminum electrolytic capacitor of 10
F or greater placed near the audio
power amplifier is recommended.
The midrail bypass capacitor (CBYP) is the most critical capacitor and serves several important functions. During
start-up or recovery from shutdown mode, CBYP 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.
Bypass capacitor (CBYP) values of 0.47-F to 1-mF ceramic or tantalum low-ESR capacitors are recommended
for the best THD and noise performance.
Increasing the bypass capacitor reduces clicking and popping noise from power on/off and entering and leaving
shutdown. To have minimal pop, CBYP should be 10 times larger than CI.
CBYP ≥ 10 × Ci
The differential input stage of the amplifier cancels any noise that appears on both input lines of the channel. To
use the TPA2000D1 EVM with a differential source, connect the positive lead of the audio source to the INP
input and the negative lead from the audio source to the INN input. To use the TPA2000D1 with a single-ended
source, ac ground the INN input through a capacitor and apply the audio single to the input. In a single-ended
input application, the INN input should be ac-grounded at the audio source instead of at the device input for best
noise performance.
The TPA2000D1 employs a shutdown mode of operation designed to reduce supply current (IDD) to the absolute
minimum level during periods of nonuse for battery-power conservation. The SHUTDOWN input terminal should
be held high during normal operation when the amplifier is in use. Pulling SHUTDOWN low causes the outputs to
mute and the amplifier to enter a low-current state, IDD(SD) = 1 A. SHUTDOWN should never be left
unconnected because amplifier operation would be unpredictable.
Copyright 2002–2008, Texas Instruments Incorporated
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