When the alternating waveform is referenced to the ground, as
shown in the figure below, the zero-cross is detected too late as the waveform rises and
too early as the waveform falls.Figure 36-3. Sine Wave Referenced to
Ground
When the waveform is referenced to VDD, as shown in the figure below,
the zero-cross is detected too late as the waveform falls and too early as the waveform
rises.Figure 36-4. Sine Wave Referenced to
VDD The actual offset time can be determined for sinusoidal waveforms of a known
frequency f using the equations shown below.
Equation 36-2. ZCD Event Offset
When the External Voltage source is referenced to ground:
Toffset=sin
When the External Voltage source is referenced to VDD:
This offset time can be compensated by adding a pull-up or pull-down
biasing resistor to the ZCD input pin. A pull-up resistor is used when the external
voltage source is referenced to ground, as shown in the figure below.
Figure 36-5. External Voltage Source Referenced
to Ground
A pull-down resistor is used when the voltage is referenced to
VDD, as shown in the figure below.
Figure 36-6. External Voltage Source Referenced
to VDD
The resistor adds a bias to the ZCD input pin so that the external voltage
source must go to zero to pull the pin voltage to the ZCPINV switching
voltage. The pull-up or pull-down value can be determined with the equations shown
below.
Equation 36-3. ZCD Pull-Up/Pull-Down
Resistor
When the External Voltage source is referenced to ground:
When the External Voltage source is referenced to VDD:
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