Voltage Detectors




Voltage detectors, also known as voltage supervisors or voltage monitors, are commonly used to monitor the voltage supplied to an electronic circuit or device, or to monitor the voltage level of a battery.

When the monitored voltage level drops below or exceeds a predetermined voltage threshold, a logic output will change its value from high to low or from low to high, depending on if the output is active high or active low. The logic output can be used to enable or disable another circuit or to reset a microcontroller.

Below we explain the basic functions and identify some of the key differences between our various voltage-detector families. 



The simplest voltage detectors have only three pins. They are designed to monitor the voltage of a single voltage rail (VIN) as shown in the diagram below.



The three series listed in the diagram are low-power and ultra-low power voltage detectors with the following key characteristics:

•    XC61C    Low detect voltage, precise, low power.
•    XC61G    Low detect voltage, precise, low power, small package.
•    XC6126    Highly precise, improved temperature characteristics, low power.
•    XC6136    Ultra-low power, keeps the correct output-logic value down-to 0.4 V supply voltage.




Active High and Active Low are common terms associated with Voltage detectors.

"Active" means the low voltage detection state of the Voltage detector, so, in other words, "Active" = "Detect".  The Active High/Low shows the output logic when its input voltage falls to the detection voltage (VDF) or lower.

Therefore a Voltage Detector which is an Active High type outputs "High" at the RESET pin at low voltage detection state, and the Active "Low" type outputs "Low" at the RESETB pin at low voltage detection state.



Some voltage detectors can monitor a separate voltage rail using an additional voltage-sense pin VSEN as shown below:



In this case the voltage on pin VIN is not being monitored.  

The three series highlighted in the diagram above offer the following key characteristics:

•    XC6118 – Low power, separate sense pin, built-in time delay.
•    XC6135 – Ultra-low power, able to detect voltages down-to 0.5 V.
•    XC6134 – High performance, ultra-small package, adjustable hysteresis, detection & release delay.



Voltage detectors are normally available with either a CMOS output or an N-channel (NMOS) open-drain output.  

These two options are discussed below.


CMOS Output:


The diagram above shows a voltage detector with CMOS output.  This type of output is often used when the supply voltage of the voltage detector (VIN) and the supply voltage of the MCU (VDD) are of a similar level, so that no level shifting is required. 

CMOS outputs offer improved noise margins and lower static power consumption compared to the open-drain solution. This is because either the PMOS or the NMOS is always off, apart from a very brief time period when switching between logic states. 


N-Channel Open-Drain Output:


The diagram above shows a typical application of a voltage detector with an N-channel open-drain output.  This type of output is used when the supply voltage (VIN) of the voltage detector and the supply voltage (VDD) of the MCU are of a different level. In this example the MCU is supplied by a separate LDO or a DC-DC converter. 

The N-Channel open-drain output is a single NMOS transistor with its source pin connected to ground and its drain pin bonded out to the output pin of the voltage detector. When this transistor is switched on, the output signal is pulled to ground (low). An external pull-up resistor is required to pull the output signal to the supply voltage value VDD (high) when the transistor is switched off. In the case when the output is pulled to ground, current will flow from VDD through the external resistor to ground. This leads to a slightly increased current consumption compared to the equivalent CMOS-output solution.



In order to prevent the output from oscillating, voltage detectors are designed to have built-in hysteresis.

With simple voltage detectors the hysteresis is often fixed, but some more complex voltage detectors like our XC6132/34 series have the possibility to adjust the size of the hysteresis (VHYS) externally. The hysteresis is the difference between the release voltage (VDR) and the detect voltage (VDF), see graph here below.



In this example the detect voltage and the release voltage have a large hysteresis. This is common in industrial or automotive solutions. For general-purpose applications, the hysteresis is often fixed at approximately 5% of the detect voltage value.

Point in the graph above indicates the point in time during start-up, when the voltage level of VIN has achieved the release-voltage level.   In this example of an active-high device, the output signal switches from low to high after a time delay of tDR (release delay time) .

At point the input voltage drops below the detect voltage threshold. After a time delay of tDF (detect delay time), the output signal switches from high to low .  



Samples of the parts mentioned in this page are readily available. Torex also has other options for voltage detectors, so please either visit the website for the part details or ask your local Torex representative for further information.