The supply voltage in telecommunications, industrial and avionics applications may vary for a number of reasons such as line and load transients; power outages; or low power. The voltage detector and monitor/reset integrated circuit (IC) provide a pre-indication of the supply voltage deviation associated with these issues to help protect the system.
Although voltage detectors and supervisor/reset ICs have the same function, they are used in different types of systems. The voltage detector monitors the voltage (eg, battery voltage) and indicates to the user that the voltage is low. Voltage detectors typically do not have delays, but they have built-in hysteresis to prevent false triggering when the voltage floats near the threshold voltage.
If the supply voltage is low, the supervisor/reset IC monitors the supply voltage and resets or shuts down another device such as a microprocessor. These devices typically have a programmable output delay to prevent the system from recovering from a reset condition before the supply voltage stabilizes.
Voltage detectors and supervisor/reset ICs have different characteristics and parameters, making it difficult to select a suitable integrated circuit for a given application. The key to selecting a voltage monitor is to understand what you need and then choose the size, package type, and price you need. This series has two parts. In the first part, I will discuss the simplest voltage detectors and some different output options. In the second part, I will introduce advanced monitoring and various functions in voltage detectors and monitors.
The easiest way to monitor voltage
For some applications, you may only need a basic, small, and easy-to-implement voltage detector or monitor/reset IC. When the supply voltage is lower than the factory-set reset threshold, these devices provide a low output voltage or reset signal. After the supply voltage rises above this threshold, the output signal remains active for a given time. Available threshold options vary by device. Typical values ​​include 2.63V, 2.93V, 3.0V, 3.08V, 4.38V, and 4.63V. The accuracy is also different; although it is usually about 3%, for some devices it can be accurate to 0.75% overheating.
The basic voltage detector or reset IC does not require any external components (except possibly a pull-up resistor at the output) and is available in a small 3-pin package. Figure 1 shows an example of a simple voltage monitor TI LM809 .
Figure 1: LM809 Typical Application Circuit
When the VCC supply voltage drops below the LM809 's reset threshold, the reset pin falls low and triggers a microprocessor reset.
Output Type: Active Low, Active High, Push-Pull and Open-Drain
Active low and active high refer to the reset type. The active low reset is high; when the supply voltage drops below the voltage reset threshold, the reset pin goes low. The active high reset is just the opposite. Reset starts low and the reset pin goes high when the supply voltage drops below the voltage reset threshold.
Push-pull and open-drain refer to the output type. Push-pull uses two transistors at the output. The top transistor turns on and sets the output high; the bottom transistor turns on and sets the output low. Open-drain uses a transistor to set the output low and uses a pull-up resistor instead of the top transistor to set the output high. Push-pull has the advantages of faster speed and lower power consumption. However, open-drain outputs can be connected together to create an OR/AND logic output configuration. Figure 3 shows the open-drain output and push-pull output.
Figure 3: Output Type
I have now introduced the basics of voltage detectors and supervisor/reset ICs. Stay tuned for Part 2 for the various functions and more sophisticated ways of monitoring the voltage.
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