Importance of Voltage Supervisors and Output Topology Selection

1 Introduction

Given the amount of variable load a system may have, the power supply is not always clean and may bounce around its typical output value. Through this rebound in power, devices called voltage monitors can provide protection against unexpected surges or power drops and increase the efficiency of electronic equipment. This makes voltage monitors a necessity in any electronic application.

Voltage monitor products available on the market are differentiated by features such as threshold selection, multi-channel monitoring options, detection accuracy, output configuration, fixed or adjustable delays and watchdog functions. In this article, I will highlight the different output configurations and review things to consider when designing with these output topologies.

2.Output configuration

Think of a monitor as an analog-to-digital converter. It detects the supply voltage (analog) and provides a flag (RESET or SENSEOUT pin), which is a digital signal. The digital signal output can use open-drain or push-pull topology.

3. Open-drain output topology

Here are some things to consider when designing with an open-drain output topology:

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The open-drain outputs provide flexibility because they can be pulled up to any voltage (within the absolute maximum) to comply with the load logic, rather than pulling the output up to the monitor's supply voltage or sense voltage. Pull-up resistors appropriately limit current and maintain low-level output voltage (VOL) and high-level output voltage (VOH) specifications.

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Multiple open-drain outputs can be connected together through a single pull-up resistor. The open-drain output can also be pulled up to any voltage that matches the load logic, providing designers with flexibility.

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The pull-up resistor cannot be too low to prevent current flowing through the open drain from damaging the monitor. When the internal n-channel metal-oxide semiconductor (NMOS) (Figure 1) is turned on, current from the pull-up resistor passes through the NMOS and is pulled to ground. We should choose the pull-up resistor based on two criteria. The first criterion is the monitor's recommended maximum reset or sense current, called IRESET or ISENSE, specified in the datasheet. If the current to ground is higher than IRESET, the monitor's internal NMOS may be damaged. The second criterion is based on the VOL requirements of the load to which the voltage monitor output is connected. Lower pull-up resistors also result in higher VOL due to increased reset/sense current.

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The pull-up resistor cannot be so high that the leakage current through the open-drain resistor at high temperatures exceeds the VOH specification in the datasheet. By adding a pull-up resistor, VOH decreases due to smaller reset or sense current, resulting in a smaller voltage drop across the internal metal-oxide-semiconductor field-effect transistor (MOSFET).

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The output rise time is determined by the pull-up resistor and the output board parasitic capacitance. For faster rise times, use smaller pull-up resistors.

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The monitor's quiescent current (Iq) does not include the current through the pull-up resistors. If the pull-up voltage is pulled from the supply, the total system Iq will increase because the supply current will also pass through the pull-up resistor. If the pull-up voltage is connected to another supply, the system Iq will be equal to the monitor Iq from the datasheet. Because the pull-up voltages can be connected to different supplies, the monitor's Iq specification does not take into account the additional output current produced by using a common supply.

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The open-drain output configuration requires an additional component, a pull-up resistor, from the output to the power supply. Without a pull-up resistor, when the internal NMOS is turned off, the output is undefined because there will be no power to pull to.

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The open-drain output can vary with the output pull-up supply, and any transient coupling depends on the pull-up resistor used. Higher pull-up resistors minimize the effects of transients from the output pull-up supply.

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Figure 1: Open-drain output uses internal N-MOSFET

4.Push -pull topology

Here are some things to consider when designing with push-pull output topology:

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The push-pull configured output switches between the monitor's supply voltage and ground without the need for external pull-up resistors. Notice how the output in Figure 2 does not use the resistor in Figure 1, and how Vpullup is not present in Figure 2. Vdd and ground are switched through 2 MOSFETS.

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The output voltage in a push-pull configuration should never exceed the monitor's supply voltage by more than 0.3V because the body diode will conduct and damage the device. The body diode can carry excessive current in forward biased mode.

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The monitor's quiescent current consists of the current through an external resistor that can be connected at the monitor output.

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It is not possible to wire or tie the push-pull outputs together as in the open-drain output topology.

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Push-pull outputs are ideal for high-speed applications because push-pull outputs do not have the additional delay caused by pull-up resistors in open-drain topologies.

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Figure 2: In push-pull output topology, p-channel MOS (PMOS) and NMOS are connected together, similar to an inverter

How to identify active low level and active high level

Different types of supervisors monitor undervoltage and overvoltage conditions and provide RESET/FLAG/POWERGOOD/SENSEOUT in active-low or active-high output topologies. The pins marked "SENSEOUT" and "POWERGOOD" are activated when the monitor detects that the supply voltage is in normal operating condition, while the pin marked "RESET" is activated when the monitor detects a fault (undervoltage or overvoltage) in the supply voltage. Activate health) status.

The overvoltage active-high monitor means that RESET is activated to a logic high whenever the supply exceeds VIT+, signaling an overvoltage condition.

Table 1 helps us identify different scenarios.

Table 1: Active-high and active-low monitors.