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MAX6748KA Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX6748KA Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 14 page where VMON_TH is the desired reset threshold voltage and VTH is the reset input threshold (1.235V). Resistors R1 and R2 can have very high values to minimize cur- rent consumption due to low leakage currents. Set R2 to some conveniently high value (500k Ω, for example) and calculate R1 based on the desired reset threshold voltage, using the following formula: R1 = R2 x (VMON_TH/VRESET IN - 1) ( Ω) The MAX6748 and MAX6749 do not monitor VCC sup- ply voltage, therefore, VCC must be greater than 1.5V to guarantee RESET IN threshold accuracy and timing performance. The MAX6748 and MAX6749 can be con- figured to monitor VCC voltage by connecting VCC to VMON_TH. Dual-Voltage Monitoring (MAX6750/MAX6751) The MAX6750 and MAX6751 contain both factory- trimmed threshold voltages and an adjustable reset threshold input, allowing the monitoring of two voltages, VCC and VMON_TH (see Figure 2). RESET is asserted when either of the voltages falls below it respective threshold voltages. Manual Reset (MAX6746/MAX6747) Many µP-based products require manual reset capabil- ity, to allow an operator or external logic circuitry to initi- ate a reset. The manual reset input (MR) can connect directly to a switch without an external pullup resistor or debouncing network. MR is internally pulled up to VCC and, therefore, can be left unconnected if unused. MR is designed to reject fast, falling transients (typically 100ns pulses) and it must be held low for a minimum of 1µs to assert the reset output. A 0.1µF capacitor from MR to ground provides additional noise immunity. After MR transitions from low to high, reset remains asserted for the duration of the reset timeout period. A manual reset option can easily be implemented with the MAX6748–MAX6751 by connecting a normally open momentary switch in parallel with R2 (Figure 3). When the switch is closed, the voltage on RESET IN goes to zero, initiating a reset. Similar to the MAX6746/ MAX6747 manual reset, reset remains asserted while the voltage at RESET IN is zero and for the reset time- out period after the switch is opened. µP Reset Circuits with Capacitor-Adjustable Reset/Watchdog Timeout Delay 8 _______________________________________________________________________________________ VCC WDI tWD tRP RESET NORMAL MODE (WDS = GND) VCC OV OV Figure 4a. Watchdog Timing Diagram, WDS = GND MAX6748 MAX6749 MAX6750 MAX6751 RESET IN GND VCC VCC VMON_TH R1 R2 Figure 3. Adding an External Manual Reset Function to the MAX6748–MAX6751 |
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