Voltage monitoring can be implemented in several ways, such as with a comparator or ADC. One compact component that provides very similar functions is an IC supervisor. These components can monitor an input voltage level and provide a switching mechanism, essentially giving the circuit a thresholding behavior.
How IC Supervisors Work
An IC supervisor is used to capture a voltage measurement from a voltage rail. They are typically rated for power rail voltages above logic levels and have wide input. When the rail voltage falls below some threshold value, the output from the IC supervisor falls to 0 V, while the output reaches a HIGH state when the voltage is above the threshold. Two typical IC supervisor circuits are shown below.
Two types of supervisors: complimentary and open drain.
The function of an IC supervisor is simple: it outputs at VCC when the VCC input is pulled HIGH, and it outputs at 0 V when the VCC input drops low. The two regions are separated by a threshold. This is essentially the type of behavior you would expect from a comparator, but with the input voltage being used as the positive terminal input.
IC supervisors have some hysteresis so that small fluctuations near threshold will not cause involuntary triggering. Because IC supervisors have limited pin availability, they typically do not have enough room to apply external passives to set the threshold or hysteresis window. Therefore, these values will be fixed and are chosen based on part number selection.
How are IC supervisors used in a practical circuit? The types of voltages they could be used to monitor include:
Monitoring a mechanical switch
Monitoring an analog indicator pin with varying voltage level
Monitoring a digital indicator pin in the presence of noise
Monitoring a voltage rail and toggling an EN pin
IC Supervisor vs. Comparator
Because an IC supervisor is just a dual-state comparator, what makes this different from a custom-designed discrete hysteresis comparator circuit? With a discrete comparator circuit, there is more flexibility to set the hysteresis window with an external resistor network. This is typically not possible with an IC supervisor; the hysteresis window for the supervisor will be built into the component and may have a fixed value. With a discrete comparator circuit, the hysteresis window can be changed to very wide values if needed, and the window size for a comparator can be much larger than that for an IC supervisor.
This means that, in general, a comparator can withstand much higher voltage fluctuations and noise. However, the IC supervisor is much easier to use and typically requires very little design effort. Therefore, if ensuring a smooth transition and latching to two states at logic levels, rather than the use of hysteresis, consider using an IC supervisor to adjust or condition a manual or electronic switch.
A simple example of an IC supervisor circuit is shown below. As is typical (see above), hysteresis is set internally in the component and the external pull-up resistor is used for current limiting. The open drain IC supervisor used in this circuit toggles the enable pin by shorting it to GND or pulling high to VDD through a current-limiting resistor.
Some of these components may have the required passive components needed to set a voltage threshold built into the component. This may not allow any toggling of hysteresis. For example, the hysteresis is set in the above example based on an internal resistor network. Some values of hysteresis that are typical are on the order of 100 mV.
Just to keep the potential for EMI low, make sure to shunt across the reset output with a capacitor. This will slow down the edge of the voltage transition, which keeps the edge rate of the output low as well. Because this creates some delay between input and output from the IC supervisor, these components should not be used in any kind of timing application.
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