Fast overcurrent and short-circuit current detection is a common requirement in circuit breakers and solid state relays (SSR), and can be implemented with a single AMC23C15 isolated window comparator as shown in Figure 8-1.

Figure 8-1 Using the AMC23C15 for Overcurrent and Short-Circuit Detection

The load current flowing through a shunt resistor (RSHUNT) connected in series with two back-to-back NMOS power switches of an SSR, and produces a voltage drop that is sensed by the AMC23C15. This voltage is compared against an adjustable threshold for overcurrent detection and a fixed, 60-mV threshold for short-circuit detection, respectively. The sense voltage can be positive or negative in respect to VSSS, which is the ground reference for the isolated comparator, depending on the direction of current flow. The absolute value of the trip threshold for overcurrent detection is set by the external resistor R1. The trip threshold for short-circuit detection is fixed by the internal 60-mV reference. Overcurrent conditions are signaled on OUT1, and short-circuit conditions are signaled on OUT2. For a detailed description and a reference design of a SSR, see the Overcurrent and Overtemperature Protection for Solid-State Relays Reference Design design guide available for download on www.ti.com.

As depicted in Figure 8-1, the integrated LDO of the AMC23C15 allows direct connection of the high-side supply input (VDD1) to the VDDM supply that is generated by the isolated switch driver TPSI3050-Q1. No additional isolated power supply or regulator is required to power the high-side of the isolated comparator, which makes the solution very space and cost efficient. The fast response time and high common-mode transient immunity (CMTI) of the AMC23C15 provide reliable and accurate operation even in high-noise environments.