Ignition cranking during startup and load dumps during shutdown are common sources of voltage transients on an automotive supply line. These undervoltage (UV) and overvoltage (OV) transients can have significant magnitudes and will damage circuits that are not designed to operate during these extremes. Specialized UV and OV protection devices have been developed to disconnect sensitive electronics from power supply transients.
The LTC4368 is an example of a specialized UV and OV protection device. It utilizes a window comparator to monitor and validate the input supply. The supply voltage is monitored by a resistive divider network connected to the UV and OV monitor pins. The window comparator output drives the gates of two N-channel MOSFETs that make or break the connection between the supply and the load.
The LTC4368’s window comparator is designed with 25 mV of hysteresis on its monitor pins to improve noise immunity. Hysteresis can prevent false MOSFET on/off switching due to ripple or other high frequency oscillations on the supply line. The 25 mV of hysteresis in the LTC4368 is equivalent to 5% of the monitor pin thresholds and is common for UV and OV protection devices.
For their own protection or to reduce ignition loading, some automotive accessory circuits must be disconnected from the supply line during startup or shutdown. Due to the large transients involved, these circuits may require more hysteresis than the LTC4368 can provide alone. For such applications, the increased hysteresis requirement can be satisfied by matching the LTC4368 with a supply monitor that has adjustable hysteresis, such as the LTC2966. Figure 1 is an example of a wide voltage range automotive circuit protector. In this circuit, the LTC2966 assumes the role of the window comparator and the LTC4368 is responsible for connecting the load to the supply.
Automotive UV/OV and Overcurrent Monitor with Circuit Protection
The solution shown in Figure 1 protects electronics that are sensitive to undervoltage, overvoltage, and overcurrent transients present on an automotive supply.
The LTC2966 monitors reverse voltage, undervoltage, and overvoltage conditions. Monitoring thresholds and hysteresis levels are configured by the resistor networks 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 inverting or noninverting with respect to the inputs via the PSA and PSB pins. In Figure 1, they are configured to be noninverting. The OUTA and OUTB outputs from the LTC2966 are pulled up to the REF pin of the LTC2966 and are fed directly to 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 and overcurrent levels. The LTC4368 decides if the load should be connected to the supply based on its overcurrent comparators as well as the monitoring information from the LTC2966. The UV, OV, and SENSE (overcurrent) pins all participate in the decision-making process. If conditions are satisfied for all three pins, then the GATE pin pulls above VOUT and the load will connect to the supply through the dual N-channel MOSFET power path. If any of the three pins become dissatisfied, the GATE pin pulls below VOUT and the load is disconnected from the supply.