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

The LTC4368 is a dedicated UV and OV protection device. It uses a window comparator to monitor and verify the input power supply. 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 open the connection between the power 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.

Some automotive accessory circuits must be disconnected from the power line during startup or shutdown to protect the circuit or reduce ignition load. Due to the large transients, these circuits may require more hysteresis (beyond the hysteresis provided by the LTC4368 alone). For these applications, the LTC4368 can be matched with a power monitor that provides adjustable hysteresis, such as the LTC2966, to meet the requirement for higher hysteresis. Figure 1 shows an example of a wide voltage range automotive circuit protector. In this circuit, the LTC2966 is used as a window comparator and the LTC4368 is responsible for connecting the load to the power supply.

Figure 1. Power path control using wide voltage monitor hysteresis.

The solution shown in Figure 1 protects electronic components that are susceptible to undervoltage, overvoltage, and overcurrent transients in the automotive power line.

The LTC2966 monitors reverse voltage, undervoltage, and overvoltage conditions. The monitoring thresholds and hysteresis levels are configured by the resistor network on the INH and INL pins, and the voltages 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 the current sense resistor R11 determines the reverse current level and the overcurrent level. The LTC4368 determines whether the load should be connected to the power supply based on its overcurrent comparator and the monitoring information provided by the LTC2966. The UV, OV, and SENSE (overcurrent) pins all factor into the decision. If all three pins meet the conditional requirements, 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 do not meet the requirements, the GATE pin is pulled below V _OUT and the load is disconnected from the power 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 the automotive ignition is powered on to start the vehicle, a startup transient is generated. In this application, Channel A of the LTC2966 is configured to detect the startup transient.

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.

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 for 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 pins, whether pulled high or low, is determined by setting the PSA and PSB pins. In this application, the input pins of the LTC4368 determine the polarity of the LTC2966 output pins. 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.

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 reverse voltage protection methods for the LTC2966: a resistor solution and a diode solution, and the choice depends 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 considered appropriately 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 a diode in series with the input can severely affect the accuracy of the voltage monitoring threshold. When using a resistor solution, a 1.96 kΩ current limiting resistor is selected to protect the LTC2966 from reverse voltage. The resistor is sized to limit the current out of the input pin to 20 mA if the input voltage is pulled below −40 V. The low value resistor results in only a few millivolts of voltage drop, so the effect of the resistor on the threshold accuracy is negligible.

Figure 6. Applying overcurrent and inrush 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 the forward overcurrent protection. R10 and C1 are the 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. The 10Ω resistors R14 and R15 are added to the MOSFET gate to prevent circuit oscillations caused by PCB layout parasitic capacitance.

Start Event

Figure 7. Complete startup waveform.

The prototype was benchmarked and the results are shown in Figure 7. Prior to ignition activation, V _IN is greater than the 10 V rising monitoring threshold configured for Channel A. The LTC4368-2 UV is pulled above its 500 mV threshold by the OUTA pin of the LTC2966, causing the power path to activate and V _IN .

During startup, the 12 V bus is pulled down to 6 V. Once the 7 V dropout monitoring threshold is exceeded, OUTA pulls down the UV pin of the LTC4368-2. The LTC4368-2 responds by pulling the GATE pin low, de-energizing 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 ripples present on the bus during startup. As a result, the switching element remains off until the startup 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, reenergizing 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 the flameout, V _IN is nominal. The power path is active and V _OUT = V _IN . During the load dump, the battery voltage is pulled up to 100 V. Immediately after exceeding the 18 V boost monitoring threshold, OUTB pulls up the 0 V pin of the LTC4368-2. In response to this, the LTC4368-2 pulls the GATE pin low, disconnecting the power path and causing V _OUT to drop to 0 V. The switching element remains disconnected until the load dump discharges to 15 V. After exceeding the 15 V buck threshold, OUTB of the LTC2966 pulls down the 0 V pin of the LTC4368-2, and after the LTC4368-2 internal recovery timer times out, the TC4368-2 turns on power to the switching element again.

Figure 10. Reverse voltage protection measurement.

Figure 10 shows the use of 1.96 kΩ resistors, which limit the current out of the LTC2966 supply pins during a reverse voltage event. The applied input voltage drops from 0 V to –40 V. The current out of _{the VINA} and _VINB pins is limited to 20 mA, and the voltages at the _VINA and _VINB pins are kept a few hundred millivolts below ground. The LTC2966 safely rides out the reverse voltage event.

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 limit.

Figure 12. Assertion of 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 is ramped up until the overcurrent protection sets. As expected, the overcurrent protection activates at 2.5 A. The LTC4368 removes the load from the power supply, V _OUT , and the load current drops to 0 V. After the LTC4368 retry timer is satisfied, the LTC4368 reconnects the power supply to the load. If the overcurrent condition disappears, the load remains connected to the power supply. Otherwise, the LTC4368 removes the load from the power supply. The retry delay can be increased by adding capacitance to the RETRY pin. If desired, V _OUT can be latched by grounding the RETRY pin . In this circuit, the retry timer is set to 250 ms. See the LTC4368 data sheet for a description of how to configure the retry timer.

Reverse overcurrent protection

Figure 13. Asserting reverse overcurrent protection.

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

In this example, there is a voltage step in V _OUT when the power supply is providing 100 mA to the load, 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 power supply. This continues until the reverse current reaches 150 mA and the reverse overcurrent comparator trips. When the reverse overcurrent comparator trips, the GATE pin is pulled low. This removes the load from the power supply, preventing the load from further back-driving the power supply. The LTC4368 holds the gate low until it detects that V _OUT has dropped 100 mV below VIN.

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