TPA2000D4DAPR datasheet(20/27 Pages) TI

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TPA2000D4

STEREO 2W CLASSD AUDIO POWER AMPLIFIER

WITH STEREO HP AMPLIFIER

SLOS337B − DECEMBER 2000 − REVISED NOVEMBER 2005

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APPLICATION INFORMATION

gain setting resistors, RF and RI for HP amplifier

The voltage gain for the TPA2000D4 headphone amplifier is set by resistors RF and RI according to equation 9.

(9)

Gain

or Gain (dB)

+ 20 log

Given that the TPA2000D4 is a MOS amplifier, the input impedance is very high. Consequently input leakage

currents are not generally a concern, although noise in the circuit increases as the value of RF increases. In

addition, a certain range of RF values is required for proper start-up operation of the amplifier. Taken together

it is recommended that the effective impedance seen by the inverting node of the amplifier be set between

5 k

Ω and 20 kΩ. The effective impedance is calculated in equation 10.

(10)

Effective Impedance

FRI

F ) RI

As an example, consider an input resistance of 20 k

Ω and a feedback resistor of 20 kΩ. The gain of the amplifier

would be − 1 and the effective impedance at the inverting terminal would be 10 k

Ω, which is within the

recommended range.

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 11.

(11)

fc +

F CF

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

For maximum signal swing and output power at low supply voltages like 1.6 V to 3.3 V, BYPASS is biased to

VDD/4. However, to allow the output to be biased at VDD/2, a resistor, R, equal to RF must be placed from the

negative input to ground.

input capacitor, CI for HP amplifier

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 12.

(12)

fc +

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 13.

(13)

I +

I fc

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/4, which is

likely higher than the source dc level. It is important to confirm the capacitor polarity in the application.

TPA2000D4DAPR Datasheet(PDF) 20 Page - Texas Instruments

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