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MAX801_CPA Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX801_CPA Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page 8-Pin µP Supervisory Circuits with ±1.5% Reset Accuracy 8 _______________________________________________________________________________________ _______________Detailed Description The MAX801/MAX808 microprocessor (µP) supervisory circuits provide power-supply monitoring and backup- battery switchover in µP systems. The MAX801 also provides program-execution watchdog functions (Figure 1). Use of BiCMOS technology results in an improved, 1.5% reset-threshold precision while keeping supply currents typically at 68µA (48µA for the MAX808). The MAX801/MAX808 are intended for bat- tery-powered applications that require high reset- threshold precision, allowing a wide power-supply operating range while preventing the system from oper- ating below its specified voltage range. RESET and RESET Outputs The MAX801/MAX808’s RESET output ensures that the µP powers up in a known state, and prevents code- execution errors during power-down and brownout conditions. It does this by resetting the µP, terminating program execution when VCC dips below the reset threshold. Each time RESET is asserted, it stays low for at least the 200ms reset timeout period (set by an inter- nal timer) to ensure the µP has adequate time to return to an initial state. The internal timer restarts any time VCC goes below the reset threshold (VRST) before the reset timeout period is completed. The watchdog timer on the MAX801 can also initiate a reset (see the MAX801 Watchdog Timer section). The RESET output is active low, and is implemented with a strong pull-down/relatively weak pull-up structure. It is guaranteed to be a logic low for 0V < VCC < VRST, pro- vided VBATT is greater than 2V. Without a backup bat- tery, RESET is guaranteed valid for VCC ≥ 1V. The RESET output is the inverse of the RESET output; it both sources and sinks current and cannot be wire-OR connected. Low-Line Comparator The low-line comparator monitors VCC with a threshold voltage typically 52mV above the reset threshold, with 13mV of hysteresis. Use LOWLINE to provide a non- maskable interrupt (NMI) to the µP when power begins to fall, initiating an orderly software shutdown routine. In most battery-operated portable systems, reserve ener- gy in the battery provides ample time to complete the shutdown routine once the low-line warning is encoun- tered and before reset asserts. If the system must con- tend with a more rapid VCC fall time (such as when the main battery is disconnected, when a DC-DC converter shuts down, or when a high-side switch is opened dur- ing normal operation), use capacitance on the VCC line to provide time to execute the shutdown routine (Figure 3). First calculate the worst-case time required for the system to perform its shutdown routine. Then, with worst-case shutdown time, worst-case load current, and minimum low-line to reset threshold (VLR(min)), VRESET VLOWLINE VCC VRESET (MAX801) VCE OUT (MAX808) VRST VLL tRP tRP VBATT SHOWN FOR VCC = 0V to 5V, VBATT = 2.8V, CE IN = GND Figure 2a. Timing Diagram, VCC Rising VRESET VLOWLINE VCC VRESET (MAX801) VCE OUT (MAX808) VRST VRST + VLR VBATT tRCE tRD tRD tLL SHOWN FOR VCC = 5V to 0V, VBATT = 2.8V, CE IN = GND Figure 2b. Timing Diagram, VCC Falling |
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