TA0105A Datasheet, PDF (24/29 Pages) Tripath Technology Inc. – STEREO CLASS-T DIGITAL AUDIO AMPLIFIER DRIVER USING DIGITAL POWER PROCESSING (DPPTM ) TECHNOLOGY

Tripath Technology, Inc. - Technical Information
OUTPUT FILTER DESIGN
One advantage of Tripath amplifiers over PWM solutions is the ability to use higher-cutoff-frequency
filters. This means load-dependent peaking/droop in the 20kHz audio band potentially caused by the filter
can be made negligible. Furthermore, speakers are not purely resistive loads and the impedance they
present changes over frequency and from speaker model to speaker model.
Tripath recommends designing the filter as a 2nd order, LC filter. Tripath has obtained good results with
L_O = 33uH and C_O = 0.22uF (resonant frequency of 59kHz). The filter capacitor must be able of sustain
the ripple current caused by the high frequency switching. Thus, a high quality film capacitor is strongly
recommended.
The typical application of the TA0105A is driving “high impedance” loads from 12.5 ohms and above.
This dictates the use of a larger value output inductor, L_O, as compared to other Tripath amplifiers to
minimize in band output filter peaking and match better to the intended load impedance.
There is a compromise between inductor value and amplifier efficiency. Tripath amplifiers count on the
inductor current making “free” transitions. Take the case where the inductor current is flowing out towards
the load. This is the case where there is a positive going output waveform. When the top side device
turns off, the output voltage will “flip” to keep the inductor current in the same direction. If the entire
transition of the output voltage (from VPP to VNN) occurs before the bottom side device is enhanced,
then the transition is free. This has a positive effect on amplifier efficiency. If the bottom side device
turns on before the transition is completed then power is wasted and the amplifier efficiency suffers. The
output transition time is directly proportional to the inductor value and the supply voltage. Thus, larger
values of inductance (for a given fet output capacitance) will result in longer transition times and
decreased efficiency for a fixed supply rail. The value of L_O, 33uH, recommended above was chosen as
a reasonable compromise between efficiency and load “damping.” An upper bound on L_O without totally
sacrificing efficiency, is 47uH for typical TA0105A supply voltages and the STW20NM50FD fets. Above
this value, the designer should fully characterize the amplifier efficiency before settling on the inductor
value. The peaking exhibited by a lightly loaded LC filter can be equalized out (to some degree) by an
input RC filter located before the input coupling capacitor, C_I. This will result in a flatter magnitude
response over a wider range of output loads. In addition, it will provide additional protection (beyond that
provided by the zobel network) against high frequency signals that can cause the output filter to resonate.
The core material of the output filter inductor has an effect on the distortion levels produced by a
TA0105A amplifier. Tripath recommends low-mu type-2 iron powder cores because of their low loss and
high linearity (available from Micrometals, www.micrometals.com). The specific core used on the RB-
TA0105A was a T106-2 wound with 49 turns of 18AWG wire.
Tripath also recommends that an RC damper be used after the LC low-pass filter. No-load operation of a
TA0105A amplifier can create significant peaking in the LC filter, which produces strong resonant currents
that can overheat the output MOSFETs and other components. The RC dampens the peaking and
prevents problems. Tripath has obtained good results with R_D = 15Ω and C_D = 0.22uF. The zobel
resistor must be able dissipate the power of the LC resonance as well as the remainder of high frequency
energy that passes through the LC filter. A typical power rating for this resistor is 10W. The zobel resistor
power capability will need to increased if the application requires full power at 20kHz. The zobel capacitor
must be able to sustain the ripple current caused by the high frequency switching. Thus, a high quality
film capacitor is recommended.
LOW-FREQUENCY POWER SUPPLY PUMPING
A potentially troublesome phenomenon in single-ended switching amplifiers is power supply pumping.
This phenomenon is caused by current from the output filter inductor flowing into the power supply output
filter capacitors in the opposite direction as a DC load would drain current from them. Under certain
conditions (usually low-frequency input signals), this current can cause the supply voltage to “pump”
(increase in magnitude) and eventually cause over-voltage/under-voltage shut down. Moreover, since
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TA0105A – RW/ Rev. 2.2/05.05