Datasheet

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
small LC filter on the V
DD
connection.
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 μF, placed as close as possible to the device V
DD
lead works best. Placing this
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
The gain-setting resistors, R
IN
and R
fb
, must be chosen so that noise, stability, and input capacitor size of the
TPA6138A2 are kept within acceptable limits. Voltage gain is defined as R
fb
divided by R
IN
.
Selecting values that are too low demands a large input ac-coupling capacitor, C
IN
. Selecting values that are too
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
INPUT RESISTOR VALUE, R
IN
FEEDBACK RESISTOR VALUE,
GAIN
R
fb
–1 V/V 10 kΩ 10 kΩ
–1.5 V/V 8.2 kΩ 12 kΩ
–2 V/V 15 kΩ 30 kΩ
–10 V/V 4.7 kΩ 47 kΩ
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.
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Product Folder Link(s): TPA6138A2