Tripath Technology, Inc. - Technical Information
A P P L I C A T I O N I N F O R M A T I O N
TA2022 Basic Amplifier Operation
The TA2022 has three major operational blocks: the signal processor, the MOSFET driver, and the
power MOSFETs. The signal processor is a 5V CMOS block that amplifies the audio input signal and
converts the audio signal to a switching pattern. This switching pattern is spread spectrum with a
typical idle switching frequency of about 650kHz. The switching patterns for the two channels are not
synchronized and the idle switching frequencies should differ by at least 40kHz to avoid increasing
the audio band noise floor. The idle frequency difference can be accomplished by offsetting the value
of CFB for each channel. Typical values of CFB are 390pF for channel 1 and 560pF for channel 2.
The MOSFET driver level-shifts the signal processor’s 5V switching patterns to the power supply
voltages and drives the power MOSFETs. The MOSFET driver includes a switching power supply
integrated to generate the VN10 supply. The VN10 supply powers the low side gate drivers as well
provides the charging current need for the “bootstrapped” supplies (VBOOT1 and VBOOT2) that
power the high side MOSFET drivers. VN10 must be stable (regulated) at 10V to 12V above VNN.
The VN10 circuitry shown in the Application / Test Circuit typically produces 11V above VNN.
The power MOSFETs are N-channel devices configured in half-bridges and are used to supply power
to the output load. The outputs of the power MOSFETs (OUT1 and OUT2) must be low pass filtered
to remove the high frequency switching pattern. A residual voltage from the switching pattern will
remain on the speaker outputs when the recommended output LC filter is used, but this signal is
outside of the audio band and will not affect audio performance.
Circuit Board Layout
The TA2022 is a power (high current) amplifier that operates at relatively high switching frequencies.
The output of the amplifier switches between VPP and VNN at high speeds while driving large
currents. This high-frequency digital signal is passed through an LC low-pass filter to recover the
amplified audio signal. Since the amplifier must drive the inductive LC output filter and speaker loads,
the amplifier outputs can be pulled above the supply voltage and below ground by the energy in the
output inductance. To avoid subjecting the TA2022 to potentially damaging voltage stress, it is critical
to have a good printed circuit board layout. It is recommended that Tripath’s layout and application
circuit be used for all applications and only be deviated from after careful analysis of the effects of any
changes. Please refer to the TA2022 evaluation board document, EB-TA2022, available on the
Tripath website, at www.tripath.com.
The following components are important to place near their associated TA2022 pins and are ranked
in order of layout importance, either for proper device operation or performance considerations.
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The capacitors CHBR provide high frequency bypassing of the amplifier power supplies and
will serve to reduce spikes across the supply rails. CHBR should be kept within 1/8” (3mm)
of the VNN(8,9) and VPP(4,12) pins. Please note that both VNN1 and VPP1 as well as
VNN2 and VPP2 must be decoupled separately. In addition, the voltage rating for CHBR
should be 100V as this capacitor is exposed to the full supply range, VPP-VNN.
DO, fast recovery PN junction diodes minimize undershoots of the outputs with respect to
power ground during switching transitions and abnormal load conditions such as output
shorts to ground. For maximum effectiveness, these diodes must be located close to the
output pins and returned to their respective VNN1(2). Please see Application/Test Circuit
for ground return pin.
CFB removes very high frequency components from the amplifier feedback signals and
lowers the output switching frequency by delaying the feedback signals. In addition, the
value of CFB is different for channel 1 and channel 2 to keep the average switching
frequency difference greater than 40kHz. This minimizes in-band audio noise.
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TA2022 – KLI/1.2/07-04
TRIPATH [ TRIPATH TECHNOLOGY INC. ]
