SBOS848B December 2017 – October 2019 INA381
The INA381 can measure current across a shunt resistor with common-mode voltage ranges from –0.3 V to +26 V. The INA381 is capable of measuring low-side current sensing allowing enough margin below ground to accurately measure current through the load. One common application for low-side current sensing is a solenoid control application. As described in Figure 52, a typical high-voltage solenoid application consists of a high-voltage NMOS transistor, a low-ohmic shunt resistor connected to the source of the NMOS transistor, and a solenoid. A solenoid is often used for applications that control a relay that triggers an on-off state. As current flows through the solenoid, the current flowing through the copper windings generate a magnetic field around the iron that can be used to open or close a relay. Industrial valves, electromechanical relays, and PLC control relays are often built of solenoids, and the driver circuitry for solenoids are designed discretely, as shown in Figure 52.
A microcontroller unit is often used to control the duty cycle of the NMOS switch to control the position of the solenoid. By controlling the duty cycle of the solenoid driver, the current flowing through the solenoid can be controlled, which in turn can be used to perform position control. However, for applications that need two states, on and off, a microcontroller can be expensive and overkill. If a solenoid is located remotely in specific application, the routing of the current-sense amplifier signal back to the microcontroller can create additional overhead and often increase the cost of the application. The INA381 has a built-in comparator that can be programmed to assert an ALERT when the CMPIN signal exceeds the CMPREF threshold signal. The ALERT signal can be used to feed the ALERT signal back to the gate driver circuitry of the NMOS, which can disable the NMOS switch to turn the circuit off to protect from damage. Effective impedance of a solenoid is an inductor in series with a resistance. If the solenoid is prone to damage, the inductor can lose inductance and behave as a shorted resistor. If not protected, high current can flow through the solenoid and damage the system, causing permanent failure. The INA381, with an ALERT pin that responds in as fast as 10 µs, can be directly connected to the NMOS driver to remove power from the solenoid in the event of an overcurrent condition. When the load current decreases to less than the safe operating limit, the ALERT clears and enables safe operation of the solenoid. This design example can be used as a guideline to implement the INA381 for a solenoid application.
Based on Equation 4, the design example for the CMPREF voltage is 4 V. The threshold voltage is set using simple resistor dividers R1 and R2. R1 is set with 2.5 kΩ, and R2 is set with 10 kΩ. This 4-V threshold is set at the CMPREF pin. When the current exceeds 4 A, voltage on VOUT exceeds 4 V, and the ALERT pin asserts a low signal indicating a fault detection. The device is configured in transparent mode by connecting the RESET pin to ground. Because of this configuration, when the current signal falls below 4 A of current, the ALERT pin is pulled high and resets the fault detection, maintaining safe operation of the solenoid. This example explains a methodology where a solenoid can be self-protected and triggered based on a set, safe-operating, current threshold.
In this application, 4 A and higher are considered overcurrent conditions and some corrective action must be taken to prevent the current from destroying the system. The INA381 offers corrective action through an ALERT pin that can be tailored for a specific overcurrent condition through the CMPREF pin. To set the proper CMPREF value, a gain option and an RSENSE value must first be determined. This design example uses a gain of 200 V/V and an RSENSE value of 5 mΩ. CMPREF is calculated according to Equation 4 in this particular case. This value is calculated to be approximately 4 V. This value can be achieved through either a voltage divider or LDO. In this particular instance, the voltage divider was chosen.