Frequently Asked Questions (FAQs) About the MAX22191 Parasitically Powered, Industrial Digital Input

Abstract: The MAX22191 is a parasitically powered, industrial, digital input IC featuring fast propagation delays and reduced power and heat dissipation. This series of frequently asked questions (FAQs) answers the most common questions about MAX22191 operation for industrial applications.

What are the key benefits of the MAX22191?

The MAX22191 offers three key benefits to the industrial design engineer.

  1. High level of integration, allowing for either a current source or sink configuration for binary sensor designs.
  2. Reduced power and heat dissipation, making it ideal for small sensors or high-density applications.
  3. Wide input voltage tolerance, allowing operation for input voltages ranging from -60V to +60V.

Can I interface to IEC 61131-2 Type 1, 2, and 3 sensors?

With an output current typically around 2.3mA, the MAX22191 supports operation with Type 1 and Type 3 binary sensors.

What does "parasitically powered" mean?

The MAX22191 operates in two modes: parasitically powered or powered from an external source applied to the VCC pin. When VCC is connected to ground, the device derives all its operating power from the signal on the IN pin. As the signal at IN rises, the device turns on and sinks or sources current as a normal digital input.

How does applying a voltage to the VCC pin affect operation of the MAX22191?

The MAX22191 OUT pin is a current source when the device is parasitically powered, or derives power from the signal at the input, IN. When at least 3V is applied to the VCC pin, the output changes from a current source to a CMOS voltage output. In this configuration, the propagation delay from IN to OUT is reduced.

Can I change the value of the REXT resistor to adjust the output current? If so, how do I calculate the new resistor value?

The MAX22191 was not designed to operate with an adjustable output current. The only value for the REXT resistor is 40.2kΩ with a ±10% tolerance. The "IN and OUT Current vs. REXT Resistance" plot in the Typical Operating Characteristics section of the IC data sheet shows how the input and output currents are affected by this resistance.

Can I connect a current-sourcing sensor to MAX22191?

Yes. The MAX22191 supports both current-sinking and current-sourcing configurations. Connect the sensor to the GND terminal of the MAX22191 for current-sourcing sensors and switches. When using the MAX22191 in a current-sinking configuration, the sensor/switch should be connected to the IN line of the device. The Typical Operating Circuits in the IC data sheet show examples of both configurations.

What is the maximum switching speed of the input the MAX22191 can handle?

The MAX22191 has a fast response time (250ns, max, propagation delay) but was designed to operate with switching speeds no faster than 500kHz. Note that any extra resistance on the input line requires a slower switching speed. The 48V circuit shown on the MAX22191 evaluation kit (EV kit) (MAX22191EVKIT), for example, has a maximum switching rate typically around 20kHz.

How do you recommend I layout my PCB for the MAX22191?

The placement of the REXT resistor is extremely important for normal operation of the MAX22191. The REXT resistor is used to generate the boost current that enables the fast IN-to-OUT propagation delays. Any parasitic capacitance or inductance between the REXT pin and the REXT resistor can create problems with the boost current, resulting in glitches at the output. Ensure that the REXT resistor is placed as close as possible to the REXT pin. Refer to the MAX22191 EV kit for a sample layout.