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UM805TE Datasheet(PDF) 9 Page - Union Semiconductor, Inc. |
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UM805TE Datasheet(HTML) 9 Page - Union Semiconductor, Inc. |
9 / 14 page ________________________________________________________________________ http://www.union-ic.com Rev.01 Jan.2015 9/14 UM805/811/812 Applications Information Negative-Going VCC Transients In addition to issuing a reset to the µP during power-up, power-down, and brownout conditions, the UM805/811/812 are relatively immune to short-duration negative-going VCC transients (glitches). Figure 3 shows typical transient duration vs. reset comparator overdrive, for which the UM805/811/812 do not generate a reset pulse. The graph was generated using a negative-going pulse applied to VCC, starting above the actual reset threshold and ending below it by the magnitude indicated (reset comparator overdrive). The graph indicates the typical maximum pulse width a negative-going VCC transient may have without causing a reset pulse to be issued. As the magnitude of the transient increases (goes farther below the reset threshold), the maximum allowable pulse width decreases. Typically, for the UM8_ _LE/ME/JE, a VCC transient that goes 125mV below the reset threshold and lasts 40µs or less will not cause a reset pulse to be issued. A 0.1µF capacitor mounted as close as possible to the VCC provides additional transient immunity. Figure 3. Maximum Transient Duration without Causing a Reset Pulse vs. Reset Comparator Overdrive Ensuring a Valid RESET ______________ Output Down to VCC = 0V When VCC falls below 1V, the UM811 RESET _____________ output no longer sinks current—it becomes an open circuit. Therefore, high-impedance CMOS-logic inputs connected to RESET _____________ can drift to undetermined voltages. This presents no problem in most applications since most µP and other circuitry is inoperative with VCC below 1V. However, in applications where RESET _____________ must be valid down to 0V, adding a pull-down resistor to RESET _____________ pin will causes any stray leakage currents to flow to ground, holding RESET _____________ low (Figure 4). R1’s value is not critical; 100kΩ is large enough not to load RESET _____________ and small enough to pull RESET _____________ to ground. A 100kΩ pull-up resistor to VCC is also recommended for the UM812 if RESET is required to remain valid for VCC < 1V. UM8_ _LE/ME/JE UM8_ _TE/SE/RE/ZE |
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