Are there suitable overvoltage and undervoltage protection devices for automotive applications?

Latest update time：2022-08-05

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Ignition cranking during startup and load dump during shutdown are common causes of voltage transients in automotive power lines. These undervoltage (UV) and overvoltage (OV) transients are large in magnitude and can cause damage to circuits that are not designed to operate under extreme conditions. Specialized UV and OV protection devices have been developed to disconnect sensitive electronic devices from power supply transients.

LTC4368 is a dedicated UV and OV protection device. It utilizes a window comparator to monitor and verify input power. The supply voltage is monitored through a resistor divider network connected to the UV and OV monitor pins. The window comparator output drives the gates of two N-channel MOSFETs to close or break the connection between the supply and the load.

The LTC4368’s window comparator provides 25 mV hysteresis on its monitor pin for improved noise immunity. Hysteresis prevents erroneous on/off switching of the MOSFET due to ripple or other high frequency oscillations in the power supply line. The 25 mV hysteresis provided by the LTC4368 is equivalent to 5% of the monitor pin threshold, which is common in UV and OV protection devices.

To protect the circuit or reduce ignition load, some automotive accessory circuits must be disconnected from the power line during startup or shutdown. Due to the presence of larger transients, these circuits may require more hysteresis (beyond what the LTC4368 alone provides). For such applications, the LTC4368 can be matched with a power supply monitor that provides adjustable hysteresis, such as the LTC2966, to meet requirements that provide higher hysteresis. Figure 1 shows an example of a wide voltage range automotive circuit protector. In this circuit, the LTC2966 acts as the window comparator and the LTC4368 connects the load to the power supply.

Figure 1. Implementing power path control using wide voltage monitor hysteresis.

Automotive UV/OV and overcurrent monitors providing circuit protection

The solution shown in Figure 1 protects electronic devices susceptible to undervoltage, overvoltage, and overcurrent transients in automotive power lines. The LTC2966 monitors reverse voltage, undervoltage, and overvoltage conditions. The monitoring threshold and hysteresis level are configured by the resistor network on the INH and INL pins, and the voltage on the RS1 and RS2 pins.

OUTA is the UV window comparator output and OUTB is the OV window comparator output. The polarity of these outputs can be selected to be inverted or non-inverted with respect to the inputs via the PSA and PSB pins. In Figure 1, they are configured non-inverting. The OUTA and OUTB outputs of the LTC2966 are pulled up to the REF pin of the LTC2966 and then fed directly into the UV and OV pins of the LTC4368.

The LTC4368 provides reverse current and overcurrent protection. The size of current sensing resistor R11 determines the reverse current level and overcurrent level. The LTC4368 determines whether the load should be connected to the supply based on monitoring information provided by its overcurrent comparator and the LTC2966. The UV, OV and SENSE (overcurrent) pins all factor into the decision. If all three pins meet the conditions, the GATE pin is pulled above V _OUT and the load is connected to the power supply through the dual N-channel MOSFET power path. If any of the three pins does not meet the requirements, the GATE pin is pulled below V _OUT and the load is disconnected from the supply.

Automotive applications powered directly from the battery are susceptible to large voltage fluctuations during engine start-stop. In this protection solution, the voltage monitoring threshold is determined by the nominal operating voltage and the expected voltage during vehicle start-up or load dump conditions, while ensuring that downstream electronics are protected.

When a car's ignition is powered to start the vehicle, a starting transient occurs. In this application, Channel A of the LTC2966 is configured to detect startup transients.

Figure 2. V _OUT vs. V _IN .

Figure 2 shows the input voltages when the power paths are active. The startup monitor Channel A is configured with a 7 V falling threshold and a 10 V rising threshold. The load dump monitor Channel B is configured with an 18 V rising threshold and a 15 V falling threshold. These thresholds were obtained by looking at different startup and load dump waveform specifications. If desired, different thresholds can be easily configured by adjusting the resistor divider string at the INL and INH inputs of the LTC2966.

Configuration

Figure 3. Resistor divider determines voltage monitor threshold.

Figure 3 shows how to calculate the resistor divider values for this application. The REF pin of the LTC2966 provides 2.404 V.

Figure 4. Range and comparator output polarity selection.

Figure 4 shows the range and output polarity configuration of this circuit. The range of each channel is selected based on the voltage range of the specific channel to be monitored. The range is configured by the RS1A/B and RS2A/B pins. The polarity of the LTC2966 output pin, whether it is pulled high or low, is determined by setting the PSA and PSB pins. In this application, the LTC4368 input pin determines the polarity of the LTC2966 output pin. For a load to be connected to the power supply, the voltage at the UV pin must be greater than 0.5 V and the voltage at the OV pin must be less than 0.5 V.

Reverse voltage protection

In the solution shown in Figure 1, both the LTC2966 and LTC4368 are protected against reverse voltage: the LTC4368 has built-in −40 V reverse voltage protection, while the LTC2966 requires the selection of a protection device.

Figure 5. Reverse voltage protection method for the LTC2966.

Figure 5 shows two methods of reverse voltage protection available for the LTC2966: a resistor solution and a diode solution, depending on the application.

In the diode solution, the diode remains active only during normal circuit operation (i.e., positive voltage). The supply current of the LTC2966 is tens of microamps, so a low power diode is sufficient, providing a compact solution. During a reverse voltage event, the diode blocks current from flowing out of the LTC2966 supply pin. The choice of diode is determined by the reverse breakdown voltage of the diode. To match the LTC4368, a 40 V diode should be selected. With the diode solution, the forward voltage drop may negatively affect the undervoltage lockout threshold and voltage monitoring threshold accuracy.

In the resistor solution, the resistor should be selected to be large enough to safely limit the current drawn from the LTC2966 supply line during a reverse voltage event. However, the size of the resistor should also be appropriately considered to ensure minimal impact on the accuracy of the undervoltage lockout and voltage monitoring thresholds. Choosing the right package size ensures that the resistor maintains safe power dissipation.

In this application, the voltage being monitored is low enough that the forward voltage of the diode in series with the input can significantly affect the accuracy of the voltage monitoring threshold. When using a resistor solution, a 1.96 kΩ current-limiting resistor can optionally be used to protect the LTC2966 from reverse voltage. If the input voltage is pulled down below −40 V, the resistor is chosen to limit the current output from the input pin to 20 mA. Low-value resistors only cause a voltage drop of a few millivolts, so the effect of the resistor on threshold accuracy is negligible.

Overcurrent and surge current protection

Figure 6. Applying overcurrent and surge current protection.

The LTC4368 is responsible for providing overcurrent and inrush current protection for the application. Figure 6 shows the relevant components. A comparator inside the LTC4368 monitors the voltage drop across the current sense resistor R11. If it is positive (V _IN to V _OUT ), the overcurrent comparator trips when the voltage from SENSE to V _OUT exceeds 50 mV. If it is negative (V _OUT to V _{IN )} , the overcurrent comparator trips when the voltage from SENSE to V _OUT exceeds –3 mV. This application uses a 20 mΩ sense resistor to set the current limit to +2.5 A and –150 mA.

Inrush current limiting allows the application to power up without asserting forward overcurrent protection. R10 and C1 are inrush current limiting devices.

In this application, the inrush current is limited to 1 A, which is well below the forward current limit of 2.5 A. C1 is selected based on the desired inrush current limit and the size of C2. R10 prevents C1 from slowing down the reverse polarity protection response, stabilizes the fast pull-down circuit, and prevents chattering during a fault condition.

The C4 capacitor is used to set the retry delay after a positive overcurrent event. The retry delay is the time the MOSFET gate remains low after an overcurrent event is detected. In this application, the retry delay is 250 ms. Add 10Ω resistors R14 and R15 to the MOSFET gate to prevent circuit oscillation caused by PCB layout parasitic capacitance.

Demo

Start event

Figure 7. Complete startup waveform.

The prototype was tested for benchmark characteristics and the results are shown in Figure 7. Before ignition is activated, V _IN is greater than the 10 V rising monitoring threshold configured for Channel A. The LTC4368-2 UV is pulled high above its 500 mV threshold by the LTC2966's OUTA pin, causing the power path to become active and V _IN .

During startup, the 12 V bus is pulled down to 6 V. Immediately after exceeding the 7 V buck monitoring threshold, OUTA pulls down the UV pin of the LTC4368-2. The LTC4368-2 responds by pulling the GATE pin low, cutting power to the switching element and causing V _OUT to drop to 0 V. The 3 V hysteresis programmed by the voltage-monitoring resistor divider allows the LTC2966 to ignore ripple that appears on the bus during startup. Therefore, the switching element remains closed until the start-up cycle is complete. At the end of the startup cycle, the battery voltage returns to its nominal value, which is greater than the 10 V threshold. The OUTA pin pulls the LTC4368-2 UV pin high, re-energizing the switching element.

Figure 8. Expanded Boot Recovery.

Figure 8 shows the startup recovery behavior. It can be seen that the LTC4368-2’s internal recovery timer (typically 36 ms) is satisfied before power is re-applied to the switching element. It can also be seen that V _IN is temporarily pulled low after power is re-applied to the switching element. This is due to charging the circuit’s load capacitance and the series input inductor. This demonstrates the need for wide voltage monitoring threshold hysteresis. This load capacitance charging transient is ignored by the LTC2966.

Figure 9. Complete load dump waveform.

Figure 9 shows the load dump behavior of the circuit. Before flameout, V _IN is nominal. The power path is activated and V _OUT = V _IN . During load dump, the battery voltage is pulled up to 100 V. Immediately after the 18 V boost monitoring threshold is exceeded, OUTB pulls up the 0 V pin of the LTC4368-2. The LTC4368-2 responds by pulling the GATE pin low, causing the power path to open and V _OUT to drop to 0 V. The switching element remains open until the load dump discharges to 15 V. After exceeding the 15 V buck threshold, the OUTB of the LTC2966 pulls down the 0 V pin of the LTC4368-2. After the LTC4368-2 internal recovery timer times out, the TC4368-2 turns on the power to the switching element again.

Figure 10. Reverse voltage protection measurement.

Figure 10 shows the use of a 1.96 kΩ resistor, which limits the current output from the LTC2966 power pin during a reverse voltage event. The input voltage of the application is reduced from 0 V to –40 V. The V _INA and V _INB pin output currents are limited to 20 mA, and the voltages at the V _INA and V _INB pins are kept several hundred millivolts below ground. The LTC2966 safely survives reverse voltage events.

Forward overcurrent protection

Figure 11 shows the inrush current limit value determined by R10 and C1. As expected, the inrush current is limited to 1 A and V _OUT is pulled up to 12 V directly without asserting the overcurrent limit.

Figure 11. Inrush current limits.

Figure 12. Asserting forward overcurrent protection and retry delay.

Figure 12 shows the LTC4368 response to a forward overcurrent event. The forward overcurrent comparator in the LTC4368 trips when the voltage between the SENSE and V _OUT pins exceeds 50 mV. The value of the current sense resistor R11 is 20 mΩ, which sets the current limit for the application to 2.5 A.

In this demonstration, the current rises steeply until the overcurrent protection asserts. As expected, the overcurrent protection activates at 2.5 A. The LTC4368 removes the load from the supply V _OUT and the load current drops to 0 V. After the LTC4368 retry timer is satisfied, the LTC4368 reconnects power to the load. If the overcurrent condition disappears, the load will remain connected to the power source. Otherwise, the LTC4368 will remove the load from the power supply. The retry delay can be increased by adding a capacitor to the RETRY pin. If desired, V _OUT can be locked by leaving the RETRY pin to ground . In this circuit, the retry timer is set to 250 ms. See the LTC4368 data sheet for retry timer configuration instructions.

Figure 13. Reverse overcurrent protection set.

Figure 13 shows the LTC4368’s response to a reverse overcurrent transient. The reverse overcurrent comparator senses the voltage between the VOUT and SENSE pins. The voltage threshold used for reverse overcurrent assertion depends on the specific product model. The LTC4368-1 sets at 50 mV and the LTC4368-2 sets at 3 mV. This application uses the LTC4368-2 model. Current sense resistor R11 is 20 mΩ. This sets the reverse overcurrent limit to 150 mA.

In this example, when the power supply supplies 100 mA to the load, there is a voltage step in V _OUT , so the value of V _OUT is greater than V _IN . As V _OUT increases, I _LOAD decreases. The voltage step is large enough to force current from the load to the source. This condition continues until the reverse current reaches 150 mA and the reverse overcurrent comparator trips. When the reverse overcurrent comparator is tripped, the GATE pin is pulled low. This removes the load from the power supply, preventing the load from driving the power supply further back. The LTC4368 will hold the gate low until it detects that V _OUT drops to 100 mV below VIN.

in conclusion

The automotive application in this article shows that the implementation of automotive protection circuits can be simplified using dedicated protection devices. The LTC2966 and LTC4368-2 are combined with minimal additional circuitry to provide accurate, reliable and comprehensive transient protection. These devices are flexible and can be configured for a variety of applications.

LTC4368

Wide operating voltage range: 2.5V to 60V
Overvoltage protection up to 100V
Reverse power protected to –40V
Bidirectional electronic circuit breaker:

+50mV positive detection threshold
–50mV reverse (LTC4368-1)
–3mV reverse (LTC4368-2)

Adjustable ±1.5% Undervoltage and Overvoltage Thresholds
Low operating current: 80μA
Low shutdown current: 5μA
Controlling back-to-back N-channel MOSFETs
Isolates 50Hz and 60Hz AC power
Hot-swappable power input
Pin-selectable overcurrent auto-retry timer or latch-off
10-Lead MSOP and 3mm x 3mm DFN Packages

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