35 ZCD - Zero-Cross Detection Module

The Zero-Cross Detection (ZCD) module detects when an A/C signal crosses through the ground potential. The actual zero-crossing threshold is the zero-crossing reference voltage, ZCPINV, which is typically 0.75V above ground.

The connection to the signal to be detected is through a series current-limiting resistor. The module applies a current source or sink to the ZCD pin to maintain a constant voltage on the pin, thereby preventing the pin voltage from forward biasing the ESD protection diodes. When the applied voltage is greater than the reference voltage, the module sinks current. When the applied voltage is less than the reference voltage, the module sources current. The current source and sink action keeps the pin voltage constant over the full range of the applied voltage. The ZCD module is shown in the following simplified block diagram.

Figure 35-1. Simplified ZCD Block Diagram

The ZCD module is useful when monitoring an A/C waveform for, but not limited to, the following purposes:

  • A/C period measurement
  • Accurate long term time measurement
  • Dimmer phase delayed drive
  • Low EMI cycle switching