The reinforced and insulated window comparator has an adjustable threshold
Comparators are one of the basic elements of analog and mixed-signal designs, providing a small but essential function, as their name suggests. Their role is often to be a “simple” safety limit switch and failure alert source – “simple” only in the sense that they do so reliably with a minimum of supporting components (or any software likely to crash).
Today, they are increasingly expected to operate in electrically noisy and environmentally harsh high voltage situations, where galvanic (ohmic) isolation is required for both basic performance of the system and for the safety of the user.
Adding such isolation to the comparator itself is difficult. It requires more than basic electrical performance, but must also be designed, built, and tested to meet several stringent industry and government standards for voltage, ruggedness, and temperature performance.
Specifications and applications
To address this need, Texas Instruments developed the AMC23C12, a reinforced, isolated window comparator with fast response time and an open-drain output that the company claims is an industry first. (Fig.1). Its input and output are separated by a certified isolation barrier to provide reinforced galvanic isolation up to 5 kVRMS according to DIN VDE V 0884-11 and UL1577, and it supports operating voltage up to 1 kVPACK. It is also highly resistant to magnetic interference.
Applications include overcurrent or overvoltage detection in motor drives, frequency inverters, solar inverters and high power DC-DC converters – all areas where voltages and currents are high, as is the risk of shock for the user, and safety-related performance is evaluated on several parameters.
The comparison window is centered around 0 V; therefore, the comparator triggers if the absolute value of the input voltage exceeds the trigger threshold value. This trigger threshold is adjustable from 20 to 300 mV via a single external resistor, the comparison window is therefore between ±20 and ±300 mV.
When the voltage on the REF pin is greater than 550 mV, the negative comparator is disabled and only the positive comparator is functional. The reference voltage in this mode can reach 2.7 V (especially useful for monitoring voltage supplies).
The open-drain output supports two modes: transparent fashion where the output follows the input state, and lock mode where the output is cleared on the falling edge of the latch input signal. The device also features high common-mode transient immunity (CMTI) of 55 kV/µs (minimum), another important consideration in these operating environments.
While it’s possible to build your own isolator/comparator combination, TI maintains that the AMC23C12 offers these less obvious advantages:
- Reduces the “size of the solution” by 50% thanks to the integration of a wide input range and low dropout regulator; a window comparator; and a precision voltage reference.
- Allows the use of smaller passive components, as the high accuracy of the worst-case trigger threshold (greater than 3%) reduces design margins.
- Provides ultra-fast overcurrent and overvoltage detection for better system protection thanks to the short response time of less than 400 ns.
- Facilitates comparator reuse across multiple designs by using a single external resistor to set the adjustable trigger threshold.
Given the mission-critical nature of many designs, even users of components that provide basic comparator functionality benefit from full technical support. The 32-page data sheet contains expected specifications for the comparator under a wide range of operating conditions, including a page calling out the specific regulatory standards it meets and under what conditions. The data sheet also contains application examples and guides, as well as a “do’s and don’ts” section, an acknowledgment of the reality of the design.
Further support is provided through the AMC23C12EVM evaluation module (Fig.2), which costs $49. Additionally, a detailed 15-page user guide provides information on connection and setup, material bill of materials, PCB layout, and more.
For those who need to do more research on approaches to fault detection in high-power systems, a brief TI blog “Meeting the Growing Needs for Fault Detection in High-Power Systems” reviews different ways to use combinations of isolated barriers, comparators and modulators. to achieve these goals, as seen in picture 3. It also has a table comparing the relative attributes of different topologies in terms of latency, accuracy, size, cost, and isolation performance.
The AMC23C12 is available in an 8-pin widebody SOIC package (5.85 × 7.50mm) and is specified over the extended industrial temperature range of -40 to +125°C.