TPA6138A2

www.ti.com

SLOS704A

– JANUARY 2011 – REVISED MAY 2011

CHARGE-PUMP FLYING CAPACITOR AND VSS CAPACITOR

The charge-pump flying capacitor serves to transfer charge during the generation of the negative supply voltage.

The VSS capacitor must be at least equal to the charge-pump capacitor in order to allow maximum charge

transfer. Low-ESR capacitors are an ideal selection, and a value of 1

μF is typical. Capacitor values that are

smaller than 1

μF can be used, but the maximum output voltage may be reduced, and the device may not

operate to specifications. If the TPA6138A2 is used in highly noise-sensitive circuits, it is recommended to add a

DECOUPLING CAPACITORS

The TPA6138A2 is a DirectPath headphone amplifier that requires adequate power-supply decoupling to ensure

that the noise and total harmonic distortion (THD) are low. A good low equivalent-series-resistance (ESR)

ceramic capacitor, typically 1

decoupling capacitor close to the TPA6138A2 is important for the performance of the amplifier. For filtering

lower-frequency noise signals, a 10-

μF or greater capacitor placed near the audio power amplifier would also

help, but it is not required in most applications because of the high PSRR of this device.

GAIN-SETTING RESISTOR RANGES

high increases the noise of the amplifier. Table 1 lists the recommended resistor values for different

inverting-input gain settings.

Table 1. Recommended Resistor Values

FEEDBACK RESISTOR VALUE,

GAIN

–1 V/V

10 k

Ω

10 k

Ω

–1.5 V/V

8.2 k

Ω

12 k

Ω

–2 V/V

15 k

Ω

30 k

Ω

–10 V/V

4.7 k

Ω

47 k

Ω

USING THE TPA6138A2 AS A SECOND-ORDER FILTER

Several audio DACs used today require an external low-pass filter to remove out-of-band noise. This is possible

with the TPA6138A2, as it can be used like a standard OPAMP. Several filter topologies can be implemented,

both single-ended and differential. In Figure 8, a multi-feedback (MFB) topology with differential input and

single-ended input is shown.

An ac-coupling capacitor to remove dc content from the source is shown; it serves to block any dc content from

the source and lowers the dc gain to 1, helping to reduce the output dc offset to a minimum.

The component values can be calculated with the help of the TI FilterPro

™ program available on the TI Web site

at:

http://focus.ti.com/docs/toolsw/folders/print/filterpro.html.

Copyright

© 2011, Texas Instruments Incorporated

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