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MAX6717AUK Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX6717AUK Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 18 page Dual/Triple, Ultra-Low-Voltage, SOT23 µP Supervisory Circuits ______________________________________________________________________________________ 11 The normal watchdog timeout period (1.12s min) begins after the first transition on WDI before the con- clusion of the long initial watchdog period (Figure 2). During the normal operating mode, the supervisor will issue a reset pulse for the reset timeout period if the µP does not update the WDI with a valid transition (high-to- low or low-to-high) within the standard timeout period (1.12s min). Leave WDI unconnected to disable the watchdog timer. The WDI unconnected-state detector uses a small (200nA typ) current source. Therefore, do not connect WDI to anything that will source more than 50nA. Power-Fail Comparator PFI is the noninverting input to a comparator. If PFI is less than VPFI (626.5mV), PFO goes low. Common uses for the power-fail comparator include monitoring prereg- ulated input of the power supply (such as a battery) or providing an early power-fail warning so software can conduct an orderly system shutdown. It can also be used to monitor supplies other than VCC1 or VCC2 by setting the power-fail threshold with a resistor-divider, as shown in Figure 3. PFI is the input to the power-fail com- parator. The typical comparator delay is 2µs from PFI to PFO. Connect PFI to ground of VCC1 if unused. Ensuring a Valid Reset Output Down to VCC = 0V The MAX6715A–MAX6729A/MAX6797A are guaranteed to operate properly down to VCC = 0.8V. In applications that require valid reset levels down to VCC = 0V, use a pulldown resistor at RST to ground. The resistor value used is not critical, but it must be large enough not to load the reset output when VCC is above the reset thresh- old. For most applications, 100k Ω is adequate. This con- figuration does not work for the open-drain outputs of the MAX6715A/MAX6717A/MAX6719A/MAX6721A/ MAX6723A/MAX6725A/MAX6727A/MAX6728A. For push- pull, active-high RST output connect the external resistor as a pullup from RST to VCC1. 1.12s MAX tWDI-NORMAL 1.12s MAX tWDI-STARTUP 35s MAX VTH VCC WDI RST tRP Figure 2. Normal Watchdog Startup Sequence MAX6728A/ MAX6729A/ MAX6797A R1 R2 PFI GND VIN PFO VTRIP = VPFI R1 + R2 R2 () MAX6728A/ MAX6729A/ MAX6797A R1 R2 PFI GND VCC VIN PFO VTRIP = R2 (VPFI) 1 R1 1 R2 +- VCC R1 [] () VPFI = 626.5mV A) B) Figure 3. Using Power-Fail Input to Monitor an Additional Power-Supply a) VIN is Positive b) VIN is Negative |
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