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. For example, protection devices can monitor the input supply using a window comparator and then validate that it is within range. Similarly, the supply voltage can be monitored by a resistive divider network connected to the UV and OV monitor pins. The window comparator output can then drive the gates of two N-channel MOSFETs that make or break the connection between the supply and the load. The window comparator can be designed with 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. For example, 25 mV of hysteresis 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 protection device can provide by itself. For such applications, the increased hysteresis requirement can be satisfied by matching the protection device with a supply monitor that has adjustable hysteresis. This article walks through how to design a wide voltage range automotive circuit protector.
Figure 1. Power path control with wide voltage monitor hysteresis
Automotive UV/OV and Overcurrent Monitor with Circuit Protection
The architecture shown in Figure 1 protects electronics that are sensitive to undervoltage, overvoltage, and overcurrent transients present on an automotive supply. Figure 1 is an example of a wide voltage range automotive circuit protector. In this circuit, an LTC4368 from Analog Devices serves as the specialized UV and OV protection device and is responsible for connecting the load to the supply. The role of the window comparator is managed by an LTC2966.
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