With the never-ending demand for improved system reliability, an increasing need exists for
load-sensing capabilities in control systems. The circuit shown implements this function by combining an economical microcontroller reset circuit with a sense resistor and a few other
components (Fig. 1).
Although not originally designed for this application, the reset circuit contains all of the required functions while consuming only 1.0 mA of current. This application uses an ON Semiconductor NCP301LSN09, which contains a precise 0.9-V reference, a comparator with hysteresis, and an output driver.
The basic function of the circuit is to monitor the current at the load. As current flows through the load, a proportional voltage drop with respect to ground appears across RSENSE where:
VSENSE = ILOAD × RSENSE
Depending upon the load-current level, the reset output will either be at a low state (0 V) or a high state (VDD) as indicated by the waveforms in Figure 2. The following conditions apply:
If:
ILOAD < VDET−/RSENSE
then:
RESET output = 0 V
If:
ILOAD ≥ (VDET− + VHYS)/RSENSE
then:
RESET output = VDD
For the NCP301LSN09, VDET− = 0.9 V, while VHYS = 0.045 V.
The load can be either resistive or inductive. This circuit also can be configured to detect the loss of a load. To accomplish this, the designer sets the value of RSENSE so that the detector threshold is exceeded under nominal load conditions. For example, when a load is missing, as in the case of an open lamp, the RESET output will assume a low state. This can be applied to any practical load.
