MAX6315US41D1-T中文资料(4)

Open-Drain SOT µP Reset CircuitMAX6315

_______________Detailed Description

Reset Output

A microprocessor’s (µP’s) reset input starts the µP in aknown state. The MAX6315 asserts reset to preventcode-execution errors during power-up, power-down, or brownout conditions. RESETis guaranteed tobe a logic-low for VCC> 1V (see the ElectricalCharacteristics table). Once VCCexceeds the resetthreshold, the internal timer keeps reset asserted for thereset timeout period (tRP); after this interval RESETgoes high. If a brownout condition occurs (monitoredvoltage dips below its programmed reset threshold),RESETgoes low. Any time VCCdips below the resetthreshold, the internal timer resets to zero and RESETgoes low. The internal timer starts when VCCreturnsabove the reset threshold, and RESETremains low forthe reset timeout period.

The MAX6315’s RESEToutput structure is a simpleopen-drain n-channel MOSFET switch. Connect a pull-up resistor to any supply in the 0V to +6V range. Selecta resistor value large enough to register a logic lowwhen RESETis asserted (see the ElectricalCharacteristics table), and small enough to register alogic high while supplying all input current and leakagepaths connected to the RESETline. A 10k pullup issufficient in most applications.

Often, the pull-up connected to the MAX6315’s RESEToutput will connect to the supply voltage monitored atthe IC’s VCCpin. However, some systems may use theopen-drain output to level-shift from the monitored sup-ply to reset circuitry powered by some other supply(Figure 1). This is one useful feature of an open-drainoutput. Keep in mind that as the MAX6315’s VCCdecreases below 1V, so does the IC’s ability to sinkcurrent at RESET. Finally, with any pullup, RESETwillbe pulled high as VCCdecays toward 0V. The voltagewhere this occurs depends on the pull-up resistor valueand the voltage to which it connects (see the ElectricalCharacteristics table).

Figure 1. MAX6315 Open-Drain RESETOutput Allows Usewith Multiple Supplies

MRhas an internal 63k pullup resistor, so it can beleft open if not used. Connect a normally open momen-tary switch from MRto GND to create a manual resetfunction; external debounce circuitry is not required. If MRis driven from long cables or if the device is usedin a noisy environment, connecting a 0.1µF capacitorfrom MRto ground provides additional noise immunity.

__________Applications Information

In addition to issuing a reset to the µP during power-up,power-down, and brownout conditions, these devicesare relatively immune to short-duration negative-goingtransients (glitches). The Typical Operating Character-isticsshow the Maximum Transient Duration vs. ResetThreshold Overdrive, for which reset pulses are notgenerated. The graph was produced using negative-going pulses, starting at VRSTmax and ending belowthe programmed reset threshold by the magnitude indi-cated (reset threshold overdrive). The graph shows themaximum pulse width that a negative-going VCCtran-sient may typically have without causing a reset pulseto be issued. As the transient amplitude increases (i.e.,goes farther below the reset threshold), the maximumallowable pulse width decreases. A 0.1µF bypasscapacitor mounted close to VCCprovides additionaltransient immunity.

Negative-Going VCCTransients

Manual-Reset Input

Many µP-based products require manual-reset capa-bility, allowing the operator, a test technician, or exter-nal logic circuitry to initiate a reset. A logic low on MRasserts reset. Reset remains asserted while MRis low,and for the reset active timeout period after MRreturns high.

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