UM805 Datasheet, PDF (9/14 Pages) Union Semiconductor, Inc. – Voltage Monitors with Manual Reset Input

UM805/811/812
Applications Information
Negative-Going V_CC 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 V_CC 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 V_CC, 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 V_CC 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 V_CC 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 V_CC provides additional transient immunity.
UM8_ _LE/ME/JE
UM8_ _TE/SE/RE/ZE
Figure 3. Maximum Transient Duration without Causing a Reset Pulse
vs. Reset Comparator Overdrive
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Ensuring a Valid RESET Output Down to V_CC = 0V
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When V_CC falls below 1V, the UM811 RESET output no longer sinks current—it becomes an open
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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
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circuitry is inoperative with V_CC below 1V. However, in applications where RESET must be valid
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down to 0V, adding a pull-down resistor to RESET pin will causes any stray leakage currents to
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flow to ground, holding RESET low (Figure 4). R1’s value is not critical; 100kΩ is large enough
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not to load RESET and small enough to pull RESET to ground.
A 100kΩ pull-up resistor to V_CC is also recommended for the UM812 if RESET is required to
remain valid for V_CC < 1V.
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http://www.union-ic.com Rev.01 Jan.2015
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