What you'll learn:
- Typical requirements for industrial LEDs.
- Designing an LED gate driver using an industrial digital input chip.
- Ruggedness characteristics of the new gate driver.
Industrial control systems universally employ 24 V DC for their field supply voltage. The presence/absence of power-supply voltage is commonly signaled by LEDs found on equipment like the controller head unit, as well as I/O and communication modules. LED supply lights aid in minimizing installation, maintenance, and downtime in industrial systems/subsystems such as programmable logic controllers (PLCs) and distributed control system (DCS) controllers, power-supply units, sensors, and actuators.
Supply LEDs should light up when defined conditions are met; for example, voltage being in the required operating supply range. Despite their conceptual simplicity, real implementations aren’t that trivial. They require voltage comparators with hysteresis, need to be self-powered, and must drive LEDs consistently.
Further requirements for such circuits include:
- Low power dissipation
- The light must be off when the supply voltage is under the minimum operating voltage
- Light intensity should be invariant of supply voltage
- The circuit should be tolerant to overvoltages
- Robustness to miswiring, such as negative voltages encountered during reverse polarity
Simple circuits can exhibit significant dependence on component, temperature, and voltage variation.
Repurposing Industrial Input Chip into LED Driver
This LED driver circuit is based on repurposing an industrial digital input chip for a function different from its originally intended application. Industrial inputs, as commonly found in industrial control systems, are based on 24-V logic and have clearly specified input current/voltage characteristics with defined input threshold voltages.
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A sinking input (occasionally called a p-type input) has a pull-down current specified to be larger than 2.0 mA at the maximum input high threshold voltage of 11 V for Type 3 inputs—the most common industrial input type. To minimize power dissipation, today’s digital input circuits limit and tightly regulate the input current to a level just slightly above the 2.0-mA limit.
LED Driver Idea for Design
Noticing that the 2.0-mA level coincides with common LED drive currents and that industrial digital inputs have clearly defined switching thresholds, it raises the question: “Would a chip like the MAX22191 single-channel digital input be suitable for power-supply monitoring and LED driving?”
This industrial input device is designed to drive an optoisolator via its OUT pin. Figure 1 shows the proposed application circuit. REXT is a current-setting resistor, enabling the OUT current to be set in the 2.0- to 2.6-mA range.
The internal current source has a tolerance of ±12.5% over temperature, input voltage, and part to part, ensuring low power dissipation and negligible light intensity variation over supply voltage. With the current-source specified OUT output voltage compliance of 5.5 V, it’s possible to drive most modern LEDs. The device’s 110-µA (typ) supply current ensures low power dissipation of the solution.
Figure 2 shows that the LED current turns on sharply at 9 V (typ) and turns off at 8 V (typ), providing 1-V hysteresis. The max-on and min-off threshold voltages are specified in the MAX22191 datasheet.
Should higher threshold voltages be required, a resistor can be placed in series with the IN input, RIN (Fig. 3). The RIN value is calculated as RIN = (threshold voltage increase)/IIN.
The LED current is flat with input voltage variation in the on state, as seen in Figure 2. This contrasts to the use of a simple bipolar/MOS transistor for driving the LED, which shows significantly higher current-voltage dependence due to the transistor’s Early voltage.
LED Control
For application cases where the LED needs to be turned on/off under logic control, the TEST input can be conveniently used, as shown in Figure 3. This allows for implementation of auxiliary functions like power-up delays, blinking, or other status feedback.
Robustness of LED Driver Design
The solution is specified for use over the –40 to +125°C operating temperature range. The IN supply voltage is tolerant to a wide voltage range of –70 to +60 V, which provides headroom for surge protection and robustness to reverse-polarity miswiring. In the event of reverse connection, the circuit has a low 1-µA input current in case of a negative input voltage.