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

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 can be of large magnitude and can cause damage to circuits not designed to operate under extreme conditions. Dedicated 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 utilizes 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 make or break the connection between the supply and the load.

The LTC4368’s window comparator provides 25 mV of hysteresis on its monitor pins 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 protected devices.

To protect the circuit or reduce the ignition load, some automotive accessory circuits must be disconnected from the power line during startup or shutdown. Due to the larger transients, these circuits may require more hysteresis (beyond the hysteresis provided by the LTC4368 alone). For these types of applications, the LTC4368 can be matched with a power supply monitor that offers adjustable hysteresis, such as the LTC2966, for higher hysteresis requirements. 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 supply.

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

Automotive UV/OV and Overcurrent Monitors for Circuit Protection

The solution shown in Figure 1 protects electronics that are 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 inverting or non-inverting with respect to the input via the PSA and PSB pins. In Figure 1, they are configured in phase. The OUTA and OUTB outputs of the LTC2966 are pulled up to the REF pins 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 overcurrent level. The LTC4368 determines whether the load should be connected to the supply based on its overcurrent comparator and monitoring information provided by the LTC2966. The UV, OV and SENSE (overcurrent) pins are all factors that affect the decision. If all three pins meet the conditions, the GATE pin is pulled to V_OUT Above, 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 to V_OUTBelow, the load is disconnected from the power supply.

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

Start transients occur when the vehicle ignition is energized to start the vehicle. In this application, Channel A of the LTC2966 is configured to detect start-up transients.

Figure 2.V_OUT with V_INrelationship curve.

Figure 2 shows the input voltage when the power path is active. The startup monitor channel A is configured with a 7 V buck threshold and a 10 V boost threshold. The load dump monitor channel B configuration is configured with an 18 V boost threshold and a 15 V buck threshold. These thresholds are obtained by looking at different startup and load dump waveform specifications. If desired, different thresholds can be easily configured by adjusting the resistor divider strings at the INL and INH inputs of the LTC2966.

configure

Figure 3. Resistor divider determines voltage monitor threshold.

Figure 3 shows how to calculate the resistor divider value 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 for 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 the load to be connected to the power supply, the voltage on the UV pin must be greater than 0.5 V and the voltage on the OV pin must be less than 0.5 V.

reverse voltage protection

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

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, depending on the application.

In the diode solution, the diode remains active only during the normal operation of the circuit (i.e. positive voltage). The LTC2966’s supply current is in the tens of microamps, so using a low-power diode is enough to provide a small-footprint solution. During a reverse voltage event, the diode blocks current flow from the LTC2966 supply pins. Which diode to choose is determined by the reverse breakdown voltage of the diode. To match the LTC4368, a 40 V diode should be chosen. With the diode solution, the forward voltage drop can negatively affect the undervoltage lockout threshold and voltage monitoring threshold accuracy.

In the resistor solution, choose a resistor large enough to safely limit the current drawn from the LTC2966 power lines during a reverse voltage event. However, due consideration is also given to the size of the resistors to ensure minimal impact on the undervoltage lockout and voltage monitoring threshold accuracy. Choosing the right package size ensures that the resistors maintain 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 seriously affect the accuracy of the voltage monitoring threshold. When using a resistive solution, an optional 1.96 kΩ current limiting resistor is used to protect the LTC2966 from reverse voltages. If the input voltage is pulled down below −40 V, the resistor size is chosen to limit the current out of the input pin to 20 mA. Low value resistors cause only a few millivolts of voltage drop, so the effect of resistors on threshold accuracy is negligible.

Overcurrent and Inrush Current Protection

Figure 6. Applying overcurrent and inrush current protection.

The LTC4368 is responsible for 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 forward (V_IN to V_OUTthe overcurrent comparator at SENSE to V_OUT open circuit when the voltage exceeds 50 mV. If it is negative (V_OUT to V_INthe overcurrent comparator at SENSE to V_OUT open circuit when the voltage 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 limit 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, well below the forward current limit of 2.5 A. C1 is selected based on the desired surge current limit value 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 in fault conditions.

Capacitor C4 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 gate of the MOSFET to prevent circuit oscillations caused by the parasitic capacitance of the PCB layout.

Demo

start event

Figure 7. Complete startup waveform.

Benchmark characterization tests were performed on the prototype, and the results are shown in Figure 7. Before ignition activation, V_IN Greater than the 10 V rise 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 VIN.

During startup, the 12 V bus is pulled down to 6 V. Immediately after the 7 V buck monitoring threshold is exceeded, 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 reducing VOUT to 0 V. The 3 V hysteresis programmed by the voltage monitoring resistor divider allows the LTC2966 to ignore ripple on the bus during startup. Therefore, the switching element remains off 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 and the switching element is powered back on.

Figure 8. Expanded Boot Recovery.

Figure 8 shows the boot recovery behavior. It can be seen that the LTC4368-2’s internal recovery timer (typically 36 ms) satisfies the requirement before powering the switching element back on. It can also be seen that after re-powering the switching element, V_IN temporarily pulled down. This is due to charging the circuit’s load capacitance and serial input inductance. This indicates 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 turning off, V_INis the nominal value. The power path is active, and V_OUT = V_IN. During load dump, the battery voltage is pulled up to 100 V. OUTB pulls up the 0 V pin of the LTC4368-2 immediately after the 18 V boost monitoring threshold is exceeded. The LTC4368-2 responds to this by pulling the GATE pin low, breaking the power path and V_OUT down to 0 V. The switching element remains open until the load dump discharges to 15 V. After the 15 V step-down threshold is exceeded, the 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 powers on the switching element again.

Figure 10. Reverse voltage protection measurement.

Figure 10 shows the use of a 1.96 kΩ resistor that limits the current out of the LTC2966 supply pins during a reverse voltage event. The applied input voltage drops from 0 V to –40 V. V_INAand V_INBThe pin output current is limited to 20 mA, V_INAand V_INBThe voltage at the pin is kept several hundred millivolts below ground. The LTC2966 is safe from reverse voltage events.

Forward overcurrent protection

Figure 11 shows the inrush current limit determined by R10 and C1. As expected, the inrush current is limited to 1 A, V_OUTPull up directly to 12 V without setting 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. When SENSE and V_OUTThe forward overcurrent comparator in the LTC4368 trips when the voltage between the pins exceeds 50 mV. The value of the current sense resistor R11 is 20 mΩ, which sets the current limit of the application to 2.5 A.

In this demonstration, the current is ramped up until the overcurrent protection is set. Overcurrent protection activates at 2.5 A, as expected. The LTC4368 removes V from the supply_OUT load, 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 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 on the RETRY pin. If desired, V can be locked by grounding the RETRY pin_OUT. In this circuit, the retry timer is set to 250 ms. See the LTC4368 data sheet for configuration instructions for the retry timer.

Figure 13. Asserting reverse overcurrent protection.

Figure 13 shows the LTC4368 response to a reverse overcurrent transient. The reverse overcurrent comparator detects V_OUT and the SENSE pin. The voltage threshold used for reverse overcurrent assertion depends on the specific product model. 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, when the power supply provides 100 mA to the load, V_OUTThere is a voltage step in , so V_OUTa value greater than V_IN. With V_OUTincrease, I_LOADdecrease. 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 inverse 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 V_OUTfalls below VIN100 mV.

in conclusion

The automotive application in this article shows 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.

LTC4368

● Wide operating voltage range: 2.5V to 60V

● Overvoltage protection to 100V

● Reverse power protection to –40V

● Two-way 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

● Controls back-to-back N-channel MOSFETs

● Can isolate 50Hz and 60Hz AC power

● Hot-swappable power input

● Pin-selectable overcurrent auto-retry timer or latch-off

● 10-pin MSOP and 3mm x 3mm DFN packages