Tripath Technology, Inc. - Preliminary Information
Input Capacitor Selection
CI can be calculated once a value for RI has been determined. CI and RI determine the input low-
frequency pole. Typically this pole is set below 10Hz to minimize attenuation at 20Hz. CIN is calculated
according to:
CI = 1 / (2π x FP x RI)
where: RI = Input resistor value in ohms (typically 20kΩ)
FP = Input low frequency pole (typically 3.6Hz)
Auxiliary Op Amp
The unused inverting op amp, as shown, is simply a voltage follower. As configured, it is not used in the
circuit. But this op amp can be used for any general purpose filtering function that requires an inverting
op amp.
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 values of RFBA, RFBB and RFBC (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.
For the best signal-to-noise ratio and lowest distortion, the modulator feedback resistors should be
adjusted to produce a differential modulator feedback voltage should be approximately 4Vpp. 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. Sometimes increasing the value of RFBC may be necessary
to achieve full power for the amplifier since the input stage will clip at approximately 4Vpp. This will
ensure that the input stage doesn’t clip before the output stage.
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 (FBK_POS and FBK_NEG) can be viewed as inputs to an inverting differential
amplifier. RFBA and RFBB bias the feedback signal to approximately 2.5V and RFBC scales the large OUTP /
OUTN signal to down to 4Vpp differentially.
TDA1400
V5
RFBA
RFBA
RFBC
FBK_POS
FBK_NEG
Processing
&
OUTP
OUTN
Modulation
RFBC
RFBB
RFBB
AGND
Figure 3: Modulator Feedback
17
TDA1400 – Rev. 0.65/KLi/02.06
TRIPATH [ TRIPATH TECHNOLOGY INC. ]
