35.3.2.3.1 Correction By Offset Current

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 35-3. Sine Wave Referenced to Ground

When the waveform is referenced to V_DD, 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 35-4. Sine Wave Referenced to V_DD

The actual offset time can be determined for sinusoidal waveforms of a known frequency

f

using the equations shown below.

Equation 35-2. ZCD Event Offset

When the External Voltage source is referenced to ground:

T_{o f f s e t} = \frac{{sin}^{-1} (\frac{Z_{C P I N V}}{V_{P E A K}})}{2 π f}

When the External Voltage source is referenced to V_DD:

T_{o f f s e t} = \frac{{sin}^{-1} (\frac{V_{D D} - Z_{C P I N V}}{V_{P E A K}})}{2 π f}

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 35-5. External Voltage Source Referenced to Ground

A pull-down resistor is used when the voltage is referenced to V_DD, as shown in the figure below.

Figure 35-6. External Voltage Source Referenced to V_DD

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 Z_CPINV switching voltage. The pull-up or pull-down value can be determined with the equations shown below.

Equation 35-3. ZCD Pull-Up/Pull-Down Resistor

When the External Voltage source is referenced to ground:

R_{p u l l u p} = \frac{R_{S E R I E S} (V_{p u l l u p} - Z_{C P I N V})}{Z_{C P I N V}}

When the External Voltage source is referenced to V_DD:

R_{p u l l d o w n} = \frac{R_{S E R I E S} (Z_{C P I N V})}{(V_{D D} - Z_{C P I N V})}