I. General Overview

️· What is it? The LTC2917/LTC2918 are microprocessor supervisory circuits. They monitor voltage rails in microprocessor-based systems and provide supervisory functions like reset generation and watchdog timer functionality. They're designed to enhance system reliability.
️· Target Application: Ideal for microprocessor-based systems needing reliable power management and fault detection. Examples include industrial controllers, communication systems, and automotive electronics.
️· Key Features: Voltage monitoring, reset generation, watchdog timer, manual reset input (LTC2918 only), selectable modes of operation.

II. Key Features and Specifications (Condensed - as much as possible from a huge data sheet!)

A. Voltage Monitoring & Reset Generation:

️· Monitored Voltage Rails: Can monitor multiple voltage rails (selectable).
️· Threshold Selectability: Threshold voltages are adjustable (using resistors).
️· Hysteresis: Built-in hysteresis to prevent oscillation.
️· Reset Output: Active-low RESET output is asserted when monitored voltage is out of range.
️· Delay: Programmable delay before reset is asserted.
️· Watchdog Timers: Both internal and external watchdog timer functionality.

B. Watchdog Timer (WDT):

️· Internal WDT: Provides automatic system refresh.
️· External WDT: Allows for more complex watchdog implementations.
️· Programmable Timeout: The timeout period is programmable.
️· Watchdog Disable Hold Current: Indicates a low current draw when the watchdog is disabled.

C. Manual Reset (LTC2918 Only):

️· Manual Reset Input: Allows external reset signals to override the automatic reset functions.
️· Pull-Up Resistor: Integrated pull-up resistor.
️· Pulse Width Requirement: Minimum pulse width required for a valid manual reset signal.

D. Operating Characteristics (Selection):

️· Supply Voltage: 2.5V to 6.0V (typically).
️· Operating Temperature: -40°C to +85°C (typical).
️· Quiescent Current: Low quiescent current to conserve power.
️· Threshold Voltage Adjustment: Adjustable threshold voltage using external resistors.
️· Reset Output Drive Capability: Drive the RESET output to a variety of loads.
️· External Components: Resistors and capacitors are used to configure the features.

E. Specific Numbers and Values (Just a Sample from a Vast List)

️· Shunt Regulation Voltage: V_CC = 6.0V (maximum pin voltage)
️· Reset Timeout Period: Programmable, dependent on capacitor values.
️· Watchdog Upper Boundary: Adjustable via capacitor.
️· Manual Reset Input Low Voltage (V_IL): 0.2 * V_CC
️· Manual Reset Input High Voltage (V_IH): 0.8 * V_CC



III. Important Notes & Cautions (From the Document)

️· Absolute Maximum Ratings: Stresses beyond these can damage the device.
️· Input Current: Be aware of the input current limitations, especially when using higher supply voltages (above 5.7V). A series dropping resistor may be needed.
️· Capacitor Selection: Capacitor values for reset delays and watchdog timeouts are critical for proper operation. Refer to the application examples and design guidelines.
️· Watchdog Implementation: For LTC2917-A/LTC2918-A, edges must simply occur before the upper boundary. For LTC2917-B/LTC2918-B, edges must occur between the lower and upper boundary.
️· Design Considerations: Refer to the application information section for detailed design guidance.