The following application note demonstrates various approaches to using the MAX6369-MAX6374 family of pin-selectable watchdog timers for general-purpose, low-power, timing functions.
The MAX6369-74 series of pin-selectable watchdog timers are designed to monitor microprocessor activity and indicate when the system is not functioning properly. During normal operation, the microprocessor should repeatedly toggle the watchdog input (WDI) before the end of the selected watchdog timeout to indicate that the system is executing correctly. If it doesn't, the monitor asserts a watchdog output (active - low WDO) to indicate that there is a problem.
This series of watchdog-only monitors is housed in a miniature SOT23-8 package with an optional watchdog timeout (7 steps 1.)7ms to 104s (TYPP)), start-up delay (7 steps 17ms to 104s (TYPP)) and output pulse width (17ms and 170ms (TYPP)) depending on the status of the selected part and 3 pins (SEL0, SEL1, SEL2).
This part can be used for general purpose timing functions, especially where current consumption is important. They are available only in the 8 A type with a maximum temperature of 20 A (-40 °C to +85 °C) from 25 to 55V power supply. When the WDI is bound to GND or V(CC), the internal timer cycles the pulse activation low when the WDO is low. These parts have several advantages over the historical "555" solutions. In addition to the lower supply current (20 A maximum for a 5V supply instead of 120 A), the entire solution occupies a much smaller board area, has a smaller package, and does not require large timing resistors and capacitors (100 F) to be used for long periods of time.
The circuit below shows a few examples.
Figure 1Use the MAX6373 to set the pulse activity low and WDO low by 170ms every 52 seconds. The load is a front-panel "power-up" LED with a 1K ohm current-limiting resistor. By pulsing LEDs instead of continuous energizing, the average current is reduced by a factor of 30 (2.)4mA), down to 88 A (80 A + 8 A IQ), indicating that the device is on without draining the battery too much. By changing the SET pins SET0 = 0V, SET1 = SET2 = V(CC), the turn-off time can be extended to 17 s, reducing the average current to 32 A (24 A + 8 A iq).
Figure 1. The LED indicator flashes to save power.
Figure 2Similar to Figure 1, but with the MAX6371 per 17 minutes (104 seconds) to turn on the load for 170 ms. The load may be a battery-powered supervisory circuit that is idle, conserves power, and then wakes up to take a measurement, then records it or sends a warning when the spec is exceeded. It could be that the water level is too high.
Figure 2Pulse load, save power.
Figure 3With the MAX6373, its set input is configured to be timer disabled. If set1 remains low longer than the watchdog period (5.)2s), then the low-level WDO pulse is low. This circuit can be used in applications where the reset button is on the front panel or is easily accessible. Button 5 must be held down deliberately2 seconds or more to trigger a reset. This prevents accidental reset when someone presses the button.
Figure 3The "safe" front panel resets.
Figure 4. Prototype diagram of LED flashing circuit.
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