TPA112D datasheet(21/28 Pages) TI

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TPA112

150-mW STEREO AUDIO POWER AMPLIFIER

SLOS212C – AUGUST 1998 – REVISED MARCH 2000

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

gain setting resistors, RF and RI (continued)

For high performance applications, metal film resistors are recommended because they tend to have lower

noise levels than carbon resistors. For values of RF above 50 kΩ, the amplifier tends to become unstable due

to a pole formed from RF and the inherent input capacitance of the MOS input structure. For this reason, a small

compensation capacitor of approximately 5 pF should be placed in parallel with RF. This, in effect, creates a

low-pass filter network with the cutoff frequency defined in equation 3.

(3)

co(lowpass) +

F CF

For example, if RF is 100 kΩ and CF is 5 pF then fco(lowpass) is 318 kHz, which is well outside the audio range.

input capacitor, CI

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 4.

(4)

co(highpass) +

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 20 kΩ and the specification calls for a flat bass response down to 20 Hz.

Equation 4 is reconfigured as equation 5.

(5)

I +

co(highpass)

In this example, CI is 0.40 µF, so one would likely choose a value in the range of 0.47 µF to 1 µF. 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 (> 10). 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 that the source dc level. It is important to confirm the capacitor polarity in the application.

power supply decoupling, CS

The TPA112 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

µF, 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

µF or greater placed near

the power amplifier is recommended.

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