Utilizes active-low output to drive high-side MOSFET input switches for system power cycling

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Utilizes active-low output to drive high-side MOSFET input switches for system power cycling

Posted Date: 2024-01-24

By Niño Angelo Pesigan, Product Applications Engineer
Ron Rogelio Peralta, Product Applications Engineer
Noel Tenorio, Product Applications Engineer


In applications such as wireless transceivers, the system is often located in remote locations and is often battery powered. Since few people can visit the site to intervene, such applications must run continuously. After a period of continued inactivity or hang, the system needs to be reset to resume operation. To achieve a system reset, cut the supply voltage, disconnect power from the system, and then connect power again to restart the system.

This article will explore what methods and techniques can be used to monitor the active low output of a circuit to drive a high-side input switch to perform a system power cycle.


To improve the reliability and robustness of electronic systems, one approach is to implement protection mechanisms that can detect faults and respond promptly. These mechanisms act like safety barriers, mitigating potential damage and ensuring the system operates properly. Power cycling ensures proper system operation and provides protection, often working when a system is unresponsive and inactive so that it can continue to function. Power cycling is accomplished with a power switch that opens and closes the path between the power input and the downstream electronic system to restart the system. Once the system's microcontroller unit (MCU) becomes unresponsive and continues to be inactive, the system enters reset mode and begins power cycling.

A more common method for implementing high-side power paths or input switches is to use MOSFETs. Either N-channel or P-channel MOSFETs can be used as input switches, each with different drive requirements. Driving an N-channel MOSFET as a high-side switch is a bit complicated, so a P-channel MOSFET is usually chosen.

Supervisory circuitry can easily detect when a system is inactive by monitoring the supply voltage and/or using a watchdog timer to detect the presence of pulses. The watchdog timer feature enhances the supervisory circuit's capabilities as a comprehensive protection solution. Once inactivity is detected, the watchdog timer asserts the reset output, which is typically an active-low signal. This signal can be used to place the microcontroller into reset mode or trigger a non-maskable interrupt, prompting the system to take corrective action. Although the active-low output is primarily used to reset the microcontroller, it is also necessary to perform a power cycle in situations such as when the system is unresponsive for an extended period of time. To this end, several techniques can be used to drive the high-side P-channel MOSFET input switch from the active low output of the supervisory circuit, resulting in better system reliability.

Use MOSFET as high side input switch

Figure 1 shows an application circuit that uses a high-side input switch to protect downstream electronic systems from power-down faults. MOSFETs enable easy selection of the appropriate voltage and current ratings based on application needs, making them ideal for high-end switching designs in systems.

Figure 1. Example of a high-side input switch implementation to protect the system from brownout faults

The high-side input switch can be an N-channel or P-channel MOSFET. When the gate voltage is low, the N-channel MOSFET switch opens and the supply voltage connection is disconnected. For an N-channel MOSFET to fully close and connect power to downstream electronic systems, the gate voltage must be higher than the supply voltage by at least equal to the MOSFET threshold voltage. Therefore, if an N-channel MOSFET is used as a high-side input switch, additional configuration circuitry, such as a charge pump, will be required. Some protection circuits also integrate comparators and charge pumps to drive high-side N-channel MOSFETs while keeping the solution simple. Using a P-channel MOSFET as the high-side input switch does not require a charge pump, but reverses the polarity. This method is simpler and therefore a common method for many applications.

Monitoring circuit output drives input switch

When using P-channel MOSFETs in a circuit, it is important to first establish proper bias conditions for the gate, source, and drain terminals. Gate-source voltage (VGS) plays a key role in controlling MOSFET conduction. For P-channel MOSFETs, the gate voltage must be lower than the source voltage, and the difference must be at least equal to the MOSFET threshold voltage. This negative bias ensures that the P-channel MOSFET is biased into its active region so that current can flow from source to drain. Additionally, the gate-source threshold voltage (VGS(th)) determines the minimum voltage required to establish a conductive path between the gate and source terminals. For P-channel MOSFETs, VGS(th) is usually specified as a negative value, indicating that the gate voltage needs to be low enough relative to the source to conduct. Another important consideration is the drain-source voltage (VDS), which is the voltage applied to the drain and source terminals. MOSFETs must operate within specified VDS limits to prevent damage to the device.

A voltage monitor or supervisory circuit can provide two options for its logic level output: active-low and active-high output signals. The former "active low level" means that when the input condition is true and is met, the output is set to low level; and when the input condition is false, the output is set to high level. The latter "active high" means that when the input condition is true, the output is set to high level; when the input condition is false and not met, the output is set to low level. Supervisory circuits are often used to reset microcontrollers, so an active-low output is used to pull the microcontroller's reset pin low during a fault. Driving a P-channel MOSFET with an active-high output is very simple, especially for an open-drain topology.

Figure 2. P-channel MOSFET is used as a high-side input switch to provide overvoltage protection.

The active-high output of the supervisory circuit is connected to the gate of the P-channel MOSFET. When the monitored voltage falls below the specified threshold, the OUT pin pulls the gate low, turning on the P-channel MOSFET. The load is therefore connected to the supply voltage. When the monitored voltage exceeds the threshold, the OUT pin goes high, the P-channel MOSFET turns off, and the load is disconnected from the supply voltage.

In Figure 2, the high-voltage adjustable timing control and monitoring circuit MAX16052 is used as an overvoltage protection circuit. The OUT pin of the device is connected directly to the gate of the P-channel MOSFET. The source of the P-channel MOSFET is connected to the input voltage and the drain is connected to the load. An external pull-up resistor is connected between VCC and the P-channel MOSFET gate to keep the gate high when the OUT pin is low.

When the monitored voltage falls below the fixed threshold specified by the MAX16052, the OUT pin pulls the gate pin low, causing the P-channel MOSFET switch to be in a short-circuit or conductive state. When the monitored voltage exceeds the threshold, the OUT pin goes high, the P-channel MOSFET turns off, and the load is disconnected from the supply voltage.

In some applications, the desired monitoring requirements may only apply to active-low outputs. This means that when the monitoring conditions are met, the output signal is low. In these cases, we must resort to techniques to control the input switches with active-low outputs. For example, if the microcontroller needs to be reset after 32 seconds of system inactivity and the system needs to enable a power cycle after 128 seconds of continuous inactivity, the watchdog timer's Watchdog Input (WDI) pin can be used to detect the inactivity. When no pulse or change is detected for a period of time (watchdog timeout period tWD), the watchdog output (WDO) goes low. The MAX16155 nanopower power supply monitor with watchdog timer is available in multiple models to meet the required watchdog timeout lengths of 32 s and 128 s. To achieve the required functionality, we need two watchdog timers, one to reset the microcontroller and another to initiate the power cycle routine shown in Figure 3. A major challenge was determining how to use the low-level outputs of different watchdog timer models to enable power cycling by opening the input switch during periods of inactivity or system unresponsiveness.

Figure 3. Using two MAX16155 watchdog timers with different watchdog timeouts, one for soft reset and one for power cycle.

NPN bipolar junction transistor used as driver circuit

One way to drive a P-channel high-side switch is to use an NPN bipolar junction transistor (BJT), as shown in Figure 4. This circuit forms an inverter that converts the active low signal from the watchdog output into the high logic signal required for P-channel MOSFET switching.

When the system is active, the watchdog output on the MAX16155 WDO pin is idle and usually high. This is then connected to the base pin of the driver transistor through a network of current limiting resistors. The normal high output of the WDO pin provides the necessary base-emitter voltage as the control input to the NPN bipolar junction transistor. It builds up enough voltage across the base-emitter junction to bring the transistor into a conducting state.

Resistor dividers are connected to the gate and source pins of the high-side MOSFET switch to control its gate-source voltage (VGS). This gate-to-source voltage determines whether the MOSFET remains on or off. When the WDO pin activates the NPN bipolar junction transistor, current flows through the transistor. This pulls the resistor divider down to GND, changing the voltage at the resistor divider junction. This voltage is then applied to the gate pin of the high side MOSFET. This creates a potential difference, with the gate pin having a lower potential than the source pin, causing the MOSFET to turn on. When the MOSFET is in the on state, power is provided to the system microprocessor or load. Figure 5 shows the current flow from the power supply through switch Q2 when the system is active.

However, when the microprocessor becomes unresponsive or fails to provide an input pulse within the predetermined timeout of the MAX16155 watchdog timer, a watchdog timeout event occurs and WDO is asserted low. Therefore, the base of NPN BJT Q1 is pulled to ground, causing it to turn off. When Q1 is turned off, the voltages at the gate and source of P-channel MOSFET Q2 will be approximately equal, which is enough to turn it off.

Figure 4. Using an NPN bipolar junction transistor (Q1) to drive a P-channel MOSFET (Q2) from an active-low output

Figure 5. Current flow during normal operation - system is active

Figure 6. Current flow during system inactivity—power cycling occurs

As shown in Figure 5, the collector pin of the NPN bipolar junction transistor is connected to a resistor divider across the high-side MOSFET. Since the NPN bipolar junction transistor is in the off state, the voltage across the resistor divider node and gate will be approximately equal to the voltage in the SOURCE pin. This will cause the potential difference between the gate and source of the MOSFET to be zero, thereby failing to meet the VGS threshold required for MOSFET Q2 to remain on. Therefore, as the MOSFET turns off, the 3.3 V supply to the microprocessor is also disconnected, effectively cutting off power to the microprocessor or load. The equivalent circuit and current flow during system inactivity and power cycling are shown in Figure 6.

When the WDO output pulse width is completed and returns to high level, the system resumes normal operation. At this stage, the microprocessor resumes sending regular input pulses to the WDI pin in case further watchdog timeout events occur. The NPN bipolar junction transistor returns to the active state, allowing the high-side MOSFET to remain on, ensuring uninterrupted power to the microprocessor or load. Figure 7 shows the waveforms during a power cycle event using an NPN bipolar transistor. As shown in CH1, no changes are detected in the WDI signal, which means the system is inactive. After the timeout period, the WDO signal in CH2 is set to low level. During this period, the high-end input switch Q1 is turned off. Therefore, no voltage is measured in CH3, there is no supply voltage to the MCU, and the system starts to reboot. CH4 is the output current drawn by the load, which goes to zero amps, indicating that the load is disconnected from the supply voltage.

Figure 7. Signals using NPN bipolar junction transistors in the drive circuit (CH1-WDI signal; CH2-WDO signal; CH3-MCU power supply; CH4-IOUT

One of the main advantages of using NPN bipolar junction transistors as high-side switch drivers is the lower cost of bipolar junction transistors. However, biasing NPN bipolar junction transistors requires proper tuning with the help of additional external components such as resistors.

Figure 8. Using an N-channel MOSFET (Q1) to drive a P-channel MOSFET (Q2) from an active-low output

N-channel MOSFET used as driver circuit

Another driver circuit using N-channel MOSFETs can be used to control high-side P-channel MOSFETs. This approach has several advantages over using bipolar transistors.

The low on-resistance of N-channel MOSFETs ensures that the voltage drop across the device is very small, resulting in lower power consumption and higher energy efficiency. The fast switching characteristics of MOSFET shorten the response time and enhance the real-time performance of the monitoring system. Another advantage of MOSFET is lower switching losses and higher operating frequency. This helps achieve smooth and efficient operation and saves power, which is very beneficial for battery-powered and similar applications.

In addition, the gate drive requirements are lower than those of bipolar junction transistors, so the drive circuit can be further simplified and the number of components required can be reduced. The watchdog output can directly drive the gate of the N-channel MOSFET shown in Figure 8. The pull-up voltage of WDO should reach the gate threshold voltage VGS(th) of the N-channel MOSFET to work properly. When the system is active, the WDO's logic high output voltage will turn on Q1, which in turn turns on Q2, supplying power to the system. As is the case with bipolar transistors, during periods of system inactivity, a logic low output from the WDO pin will turn off Q1 and disconnect Q2, thus cutting off the system's supply voltage. When using an N-channel MOSFET as the driver circuit, the signal behavior during power cycling is shown in the waveform captured in Figure 9.

The high-end switch driving approach discussed in this article is not only beneficial for wireless transceivers, but also for other applications that require system protection through power cycling routines during faults, such as overvoltage and overcurrent conditions in functional and intrinsically safe systems. helpful. The detection level depends on the conditions required for a power cycle to occur, and can be either a voltage monitor to detect voltage faults, a current sensor to prevent overcurrent, or other technologies. This article discusses how to use sensors and power monitors with active-low outputs to implement power cycling to protect downstream systems.

Figure 9. Signals using N-channel MOSFET in the drive circuit (CH1-WDI signal; CH2-WDO signal; CH3-MCU power supply; CH4-IOUT

in conclusion

There are many technologies on the market that use an active low signal from a supervisory circuit to drive a high-side switch to cycle power. NPN bipolar transistors with additional components are a lower-cost option for driving P-channel MOSFET input switches. On the other hand, the N-channel MOSFET solution requires fewer components and is easier to implement, but the overall cost is higher. N-channel MOSFETs also show many advantages when used as high-frequency switches. Both methods are well proven and can benefit system power cycling designs.

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About the author

Niño Angelo Pesigan joined Analog Devices in June 2022 as a product applications engineer for MMP-East. He holds a bachelor's degree in electrical engineering from the University of Santo Tomas, Manila, Philippines. Niño previously worked on the Customer Power Solutions team and now focuses on supporting high-performance monitoring products and their functional safety compliance.

Ron is an applications engineer who joined ADI in September 2021. Graduated from the University of the Philippines Diliman in 2020 with a bachelor's degree in electronics and communications engineering.

Noel Tenorio is a product applications engineer at ADI Philippines, focusing on high-performance monitoring products. He joined Analog Devices in August 2016. Prior to joining Analog Devices, he worked for six years as a design engineer at a switch-mode power supply R&D company. He holds a bachelor's degree in electronics and communications engineering from the National University of the Philippines Batangas, a graduate degree in electrical engineering with a specialization in power electronics, and a master of science degree in electrical engineering from Mapua University.

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