SBAA542 March 2022 AMC23C10

**Design Goals**

High Side Supply | Input Voltage |
Working Voltage |
Low Side Supply |
Output Voltage |
---|---|---|---|---|

12 V | ±170 V | ≥ 400 V | 3.3 V to 5.0 V ±10% | ≤ Low-Side Supply |

**Design Description**

A zero-crossing detector circuit changes output state when the AC input crosses the zero-cross reference voltage. This design features a single chip solution for zero-crossing detection of an AC sine wave with inverting and non-inverting digital outputs. The circuit is created by setting the comparator inverting input to ground and applying a clamped sine wave to the noninverting input. The input voltage is clamped by R1 and a pair of antiparallel diodes. In this case, diodes are used instead of an attenuator to maximize the slew rate of the input near the zero-crossing, which reduces output latency. The circuit is used for AC line zero-cross detection in control circuits to reduce standby and off-mode power consumption.

**Design Notes**

- The circuit must be capable of handling 750-V working voltage across the isolation barrier.
- The maximum input voltage at IN+ must be ±1 V
- Inverting and non-inverting output are desired
- Maximum current flowing through R1 is 100 µA ±10%
- Limit the operating voltage of each resistor in the string to 100 V ±10% maximum
- The input AC source voltage is 120
V
_{RMS}, higher AC voltages are easily accommodated with component modifications. See the Alternate Design section for details - Ensure the hysteresis voltage at the AC zero-cross is no more than ±30 mV

**Design Steps**

- Determine the ideal R1 resistor value. The
maximum peak input voltage of 120 V
_{RMS}× √2 = 170 V_{PK}. Note that the forward voltage of the diode D1 is near zero, and not included in this calculation.$R1=\frac{170\mathrm{\hspace{0.17em}}{V}_{PK}}{100\mathrm{}\mathrm{\mu}A}=1\mathrm{.}70\mathrm{}M\mathrm{\Omega}$ - Divide R1 into 3 equal resistors to maintain design limits of ≤ 100 V per resistor:
$R1=\frac{1.70\mathrm{}M\mathrm{\Omega}}{3}=566\mathrm{.}66\mathrm{}k\mathrm{\Omega}$
- Use the Analog Engineer’s Calculator to find a standard E96 1% resistor value for R1. The nearest value is 569 kΩ.
- Select the anti-parallel diodes. Choose diodes which will provide at least ± 350-mV forward voltage with the 100 µA supplied through R1.
- Optional – design low-pass filter at VINP defined
by R2 and C1. The frequency response is defined as:${F}_{C}=\frac{1}{2\pi \times R2\times C1}$

**Revised Design**

The following schematic shows implementation of the revised design using the AMC23C10.

The AMC23C10 uses capacitive isolation to provide a working voltage of 1000 V. The voltage source for VDD1 is specified from 3 V to 27 V, controlled internally through an LDO. VDD2 is specified from 2.7 V to 5.5 V. The input voltage range under normal operation is ±1 V. The logic output on OUT1 is open drain which can be used with a pullup resistor to VDD1. OUT2 is a push-pull type output needing no external pullup resistors.

**Design
Simulations**

**Measured Response**

The following images show the measured response of the zero-crossing detection circuit using the AMC23C10 isolated comparator. The input is captured on trace 1, while OUT1 and OUT2 are shown on traces 2 and 3 respectively. When measured at both the rising and falling edges of the input, the delay between the zero-crossing of the input and the output transition does not exceed 220 ns.

**Design References**

See *Analog Engineer's Circuit Cookbooks* for TI's comprehensive circuit library.

Texas Instruments, *AMC23C10 Fast Response, Reinforced Isolated Comparator
With Dual Output* data sheet

**Design Featured Isolated Comparator**

AMC23C10 | |
---|---|

Working Voltage | 1000
V_{RMS} |

VDD1 | 3.0 V–27 V |

VDD2 | 2.7 V–5.5 V |

Input Voltage Range | ±1000 mV |

Output Options | OUT1 - Open Drain |

OUT2 - Push-Pull | |

AMC23C10 |

**Alternate Design for 230-VAC
Input**

AMC23C10 | |
---|---|

Working Voltage | 1000
V_{RMS} |

AC Input | 325
V_{pk} |

R1 Ideal | 3.25 MΩ |

R1 E96 Standard | Three each 1.09 MΩ |