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The LTC®2917-A/LTC2917-B and LTC2918-A/LTC2918-B are low voltage single-supply monitors with selectable thresholds and an adjustable watchdog timer. The parts operate from 1.5V to 5.5V and consume a quiescent current of only 30μA. Two three state inputs select one of nine internally programmed thresholds without the need for external resistors. For the LTC2917, an additional three state input determines the tolerance (–5%, –10%, –15%). The tolerance for the LTC2918 is fi xed at –5%. Threshold accuracy is guaranteed at ±1.5% over the entire operating temperature range. Glitch fi ltering ensures reliable reset operation without false triggering.
The reset timeout and the watchdog timeout may be set with no external components, or adjusted using an external capacitor. A windowed watchdog feature is available for high-reliability applications (B1 versions). A separate manual reset input on the LTC2918-A/LTC2918-B allows a simple push button interface. Operation to 125°C makes the LTC2917-A/LTC2917-B and LTC2918-A/LTC2918-B suitable for automotive applications.
1. APPLICATIONS INFORMATION
LTC2917-B/LTC2918-B
For applications in which reliability is even more critical,the LTC2917-B/LTC2918-B implements a windowed watchdog function by adding a lower boundary condition to the standard watchdog function. If the WDI input receives a falling edge prior to the watchdog lower boundary, the part considers this a watchdog failure, and asserts RST low (releasing again after the reset timeout period as described above). The LTC2917-B/LTC2918-B WDI input only responds to falling edges.
Setting the Watchdog Timeout Period The watchdog timeout period is adjustable and can be optimized for software execution. The watchdog timeout period is adjusted by connecting a capacitor between WT and ground. Given a specifi ed watchdog timeout period,
the capacitor is determined by: CWT = tWD • 13.8 [nF/s]
For example, using a standard capacitor value of 0.047μF would give a 3.4s watchdog timeout period.
Leaving WT open with no external capacitor generates a timeout of approximately 3.2ms. Shorting WT to VCC generates a timeout of approximately 1.6s. Connecting WT to GND disables the watchdog function.
2.APPLICATIONS INFORMATION
Threshold Accuracy
The trip threshold for the supplies monitored is selected by confi guring the three-state input pins. When using the adjustable input, a external resistive divider sets the trip threshold, allowing the user complete control over the trip point. Selection of this trip voltage is crucial to the reliability of the system.
Any power supply has some tolerance band within which it is expected to operate (e.g. 5V±10%). It is generally undesirable that a supervisor issue a reset when the power supply is inside this tolerance band. Sucha“nuisance”reset reduces reliability by preventing the system from functioning under normal conditions.
To prevent nuisance resets, the supervisor threshold must be guaranteed to lie outside the power supply tolerance band. To ensure that the threshold lies outside the power supply tolerance range, the nominal threshold must lie outside that range by the monitor’s accuracy specifi cation.
All 27 of the selectable thresholds have the same relative threshold accuracy of ±1.5% of the programmed nominal input voltage (over the full operating temperature range).
Consider the example of monitoring a 5V supply with a 10% tolerance. The nominal threshold internal to the LTC2917 is 11.5% below the 5V input at 4.425V. With ±1.5% accuracy, the trip threshold range is 4.425V±75mV over
temperature (i.e. 10% to 13% below 5V). The monitored system must thus operate reliably down to 4.35V or 13% below 5V over temperature.
