TA0105A Datasheet, PDF (17/29 Pages) Tripath Technology Inc. – STEREO CLASS-T DIGITAL AUDIO AMPLIFIER DRIVER USING DIGITAL POWER PROCESSING (DPPTM ) TECHNOLOGY

Tripath Technology, Inc. - Technical Information
INPUT CAPACITOR SELECTION
C_I can be calculated once a value for R_IN has been determined. C_I and R_I determine the input low-
frequency pole. Typically this pole is set at 10Hz. C_I is calculated according to:
C_I = 1 / (2π x F_P x R_I)
where: R_I = Input resistor value in ohms
F_P = Input low frequency pole (typically less than 10Hz)
MODULATOR FEEDBACK DESIGN
The modulator converts the signal from the input stage to the high-voltage output signal. The optimum
gain of the modulator is determined from the maximum allowable feedback level for the modulator and
maximum supply voltages for the power stage. Depending on the maximum supply voltage, the feedback
ratio will need to be adjusted to maximize performance. The value of R_FB, in conjunction with resistors
internal to the TA0105A hybrid, (see explanation below) define the gain of the modulator. Once these
values are chosen, based on the maximum supply voltage, the gain of the modulator will be fixed even as
the supply voltage fluctuates due to current draw.
For the best signal-to-noise ratio and lowest distortion, the maximum modulator feedback voltage should
be approximately 4.5Vpp. This will keep the gain of the modulator as low as possible and still allow
headroom so that the feedback signal does not clip the modulator feedback stage. It should be noted that
the modulator works over basically a 2:1 supply voltage ratio with optimum performance around 3.5Vpp-
4Vpp of feedback. Thus, the actual value of R_FB may need to be adjusted from the typical value (39.2kΩ)
shown in the Application/Test Circuit to achieve maximum performance.
Figure 3 shows how the feedback from the output of the amplifier is returned to the input of the modulator.
The input to the modulator (FDBKN1/GNDKELVIN1 for channel 1) can be viewed as inputs to an inverting
differential amplifier. The internal 1kΩ and 1.02kΩ resistors bias the feedback signal to approximately
2.5V and R_FB, along with the internal series 1kΩ, scales the large output1 signal to down to approximately
4Vpp, depending on the supply voltage, VPP and VNN.
1/2 TA0105A
V5
1.02K 1.02K
Processing
&
Modulation
1.0K
FDBKN1
1.0K _GNDKELVIN1
RFB
RFB
OUTPUT 1
OUTPUT 1
GROUND
1.02K 1.02K
AGND
Figure 3: Modulator Feedback
17
TA0105A – RW/ Rev. 2.2/05.05