參數(shù)資料
型號(hào)： TPA4861DRG4
廠商： TEXAS INSTRUMENTS INC
元件分類： 音頻／視頻放大
英文描述： 1.1 W, 1 CHANNEL, AUDIO AMPLIFIER, PDSO8
封裝： GREEN, PLASTIC, MS-012AA, SOIC-8
文件頁(yè)數(shù)： 9/26頁(yè)
文件大小： 623K
代理商： TPA4861DRG4
www.ti.com
f
co(lowpass) +
1
2p R
F CF
(7)
Input Capacitor, CI
f
co(highpass) +
1
2p R
I
C
I
(8)
C
I +
1
2p R
I fco
(9)
Power Supply Decoupling, CS
Midrail Bypass Capacitor, CB
1
C
B
25 k
v
1
C
I RI
(10)
TPA4861
SLOS163C – SEPTEMBER 1996 – REVISED JUNE 2004
For example if RF is 100 k and CF is 5 pF, then fco is 318 kHz, which is well outside of the audio range.
In the typical application, an input capacitor, CI, is required to allow the amplifier to bias the input signal to the
proper dc level for optimum operation. In this case, CI and RI form a high-pass filter with the corner frequency
determined in Equation 8.
The value of CI is important to consider, as it directly affects the bass (low-frequency) performance of the circuit.
Consider the example where RI is 10 k and the specification calls for a flat bass response down to 40 Hz.
Equation 8 is reconfigured as Equation 9.
In this example, CI is 0.40 F; so, one would likely choose a value in the range of 0.47 F to 1 F. A further
consideration for this capacitor is the leakage path from the input source through the input network (RI, CI) and
the feedback resistor (RF) 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. Note that it is important to confirm the capacitor polarity in the application.
The TPA4861 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 VDD lead, works best. For filtering
lower-frequency noise signals, a larger aluminum electrolytic capacitor of 10 F or greater placed near the power
amplifier is recommended.
The midrail bypass capacitor, CB, serves several important functions. During start-up or recovery from shutdown
mode, CB determines the rate at which the amplifier starts up. This helps to push the start-up pop noise into the
subaudible range (so slow it cannot be heard). 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. The capacitor is fed from a 25-k
source inside the amplifier. To keep the start-up pop as low as
possible, the relationship shown in Equation 10 should be maintained.
As an example, consider a circuit where CB is 0.1 F, CI is 0.22 F and RI is 10 k. Inserting these values into
the Equation 10, we get 400
≤ 454 which satisfies the rule. Bypass capacitor, C
B, values of 0.1-F to 1-F
ceramic or tantalum low-ESR capacitors are recommended for the best THD and noise performance.
17
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