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MAX6367LKA46-T Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX6367LKA46-T Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 15 page SOT23, Low-Power µP Supervisory Circuits with Battery Backup and Chip-Enable Gating 10 ______________________________________________________________________________________ Watchdog Input (MAX6366 Only) The watchdog monitors µP activity through the watch- dog input (WDI). If the µP becomes inactive, reset asserts. To use the watchdog function, connect WDI to a bus line or µP I/O line. A change of state (high to low, low to high, or a minimum 100ns pulse) resets the watchdog timer. If WDI remains high or low for longer than the watchdog timeout period (tWD), the internal watchdog timer runs out and a reset pulse is triggered for the reset timeout period (tRP). The internal watchdog timer clears whenever reset asserts or whenever WDI sees a rising or falling edge. If WDI remains in either a high or low state, a reset pulse asserts periodically after every tWD (Figure 2). Leave WDI unconnected to dis- able the watchdog function. BATT ON Indicator (MAX6367 Only) BATT ON is a push-pull output that drives high when in battery-backup mode. BATT ON typically sinks 3.2mA at 0.1V saturation voltage. In battery-backup mode, this terminal sources approximately 10µA from OUT. Use BATT ON to indicate battery-switchover status or to supply base drive to an external pass transistor for higher current applications (Figure 3). RESET IN Comparator (MAX6368 Only) RESET IN is compared to an internal 1.235V reference. If the voltage at RESET IN is less than 1.235V, reset asserts. Use the RESET IN comparator as an undervolt- age detector to signal a failing power supply or as a secondary power-supply reset monitor. To program the reset threshold (VRTH) of the secondary power supply, use the following (see Typical Operating Circuit): VRTH = VREF (R1 / R2 + 1) where VREF = 1.235V. To simplify the resistor selection, choose a value for R2 and calculate R1: R1 = R2 [(VRTH / VREF) - 1] Since the input current at RESET IN is 25nA (max), large values (up to 1M Ω) can be used for R2 with no significant loss in accuracy. For example, in the Typical Operating Circuit, the MAX6368 monitors two supply voltages. To monitor the secondary 5V logic or analog supply with a 4.60V nominal programmed reset thresh- old, choose R2 = 100k Ω, and calculate R1 = 273kΩ. Reset Output A µP’s reset input starts the µP in a known state. The MAX6365–MAX6368 µP supervisory circuits assert a reset to prevent code-execution errors during power- up, power-down, and brownout conditions. RESET is guaranteed to be a logic low or logic high, depending on the device chosen (see Ordering Information). RESET or RESET asserts when VCC is below the reset threshold and for at least 150ms (tRP) after VCC rises above the reset threshold. RESET or RESET also asserts when MR is low (MAX6365) and when RESET IN is less than 1.235V (MAX6368). The MAX6366 watch- dog function will cause RESET (or RESET) to assert in pulses following a watchdog timeout (Figure 2). Applications Information Operation Without a Backup Power Source The MAX6365–MAX6368 provide battery-backup func- tions. If a backup power source is not used, connect BATT to GND and OUT to VCC. Watchdog Software Considerations One way to help the watchdog timer monitor the soft- ware execution more closely is to set and reset the watchdog at different points in the program rather than pulsing the watchdog input periodically. Figure 4 shows a flow diagram in which the I/O driving the Figure 2. MAX6366 Watchdog Timeout Period and Reset Active Time RESET WDI tRP tRP tWD tWD = WATCHDOG TIMEOUT PERIOD tRP = RESET TIMEOUT PERIOD tWD |
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