Datasheet

C
IN IN
1
f
2 R C
=
p
IN
C IN
1
C
2 f R
=
p
TPA6141A2
www.ti.com .................................................................................................................................................... SLOS634A –MARCH 2009–REVISED AUGUST 2009
RF AND POWER SUPPLY NOISE IMMUNITY
The TPA6141A2 employs a new differential amplifier architecture to achieve high power supply noise rejection
and RF noise rejection. RF and power supply noise are common in modern electronics. Although RF frequencies
are much higher than the 20 kHz audio band, signal modulation often falls in-band. This, in turn, modulates the
supply voltage, allowing a coupling path into the audio amplifier. A common example is the 217 Hz GSM
frame-rate buzz often heard from an active speaker when a cell phone is placed nearby during a phone call.
The TPA6141A2 has excellent rejection of power supply and RF noise, preventing audio signal degradation.
INPUT COUPLING CAPACITORS
Input coupling capacitors block any dc bias from the audio source and ensure maximum dynamic range. Input
coupling capacitors also minimize TPA6141A2 turn-on pop to an inaudible level.
The input capacitors are in series with TPA6141A2 internal input resistors, creating a high-pass filter. Equation 8
calculates the high-pass filter corner frequency. The input impedance, R
IN
, is dependent on device gain. Larger
input capacitors decrease the corner frequency. See the Operating Characteristics table for input impedance
values.
(8)
For a given high-pass cutoff frequency, the minimum input coupling capacitor is found as:
(9)
Example: Design for a 20 Hz corner frequency with a TPA6141A2 gain of +6 dB. The Operating Characteristics
table gives R
IN
as 13.2 kΩ. Equation 9 shows the input coupling capacitors must be at least 0.6 μF to achieve a
20 Hz high-pass corner frequency. Choose a 0.68 μF standard value capacitor for each TPA6141A2 input (X5R
material or better is required for best performance).
Input capacitors can be removed provided the TPA6141A2 inputs are driven differentially with less than ±1 V
RMS
and the common-mode voltage is within the input common-mode range of the amplifier. Without input capacitors
turn-on pop performance may be degraded and should be evaluated in the system.
CHARGE PUMP FLYING CAPACITOR AND HPVSS CAPACITOR
The TPA6141A2 uses a built-in charge pump to generate a negative voltage supply for the headphone
amplifiers. The charge pump flying capacitor connects between CPP and CPN. It transfers charge to generate
the negative supply voltage. The HPVSS capacitor must be at least equal in value to the flying capacitor to allow
maximum charge transfer. Use low equivalent-series-resistance (ESR) ceramic capacitors (X5R material or
better is required for best performance) to maximize charge pump efficiency. Typical values are 1 μF to 2.2 μF
for the HPVSS and flying capacitors. Although values down to 0.47 μF can be used, total harmonic distortion
(THD) will increase.
OPERATION WITH DACs AND CODECs AND INPUT RF NOISE REJECTION
When using amplifiers with CODECs and DACs, sometimes there is an increase in the output noise floor from
the audio amplifier. This occurs when the output out–of–band noise of the CODEC/DAC folds back into the audio
frequency due to the limited gain bandwidth product of the audio amplifier. Single–ended RF noise can also fold
back into the audio band thus degrading the audio signal even further
The TPA6141A2 has a built-in low-pass filter to reduce CODEC/DAC out–of–band noise and RF noise, that
could fold back into the audio frequency.
Copyright © 2009, Texas Instruments Incorporated 15
Product Folder Link(s): TPA6141A2