UM805 Datasheet, PDF (8/14 Pages) Union Semiconductor, Inc. – Voltage Monitors with Manual Reset Input

UM805/811/812
high.
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If a brownout condition occurs (V_CC dips below the reset threshold), RESET goes low. Any time
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V_CC goes below the reset threshold, the internal timer resets to zero, and RESET goes low. The
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internal timer starts after V_CC returns above the reset threshold, and RESET remains low for the
reset timeout period.
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The manual reset input (M R) can also initiate a reset. See the Manual Reset Input section.
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The UM812 has an active-high RESET output that is the inverse of the UM805/811’s RESET
output. The UM805 uses an open-drain output, and the UM811/812 have a push-pull output stage.
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Connect a pull-up resistor on the UM805’s RESET output to any supply between 0 and 6V.
Manual Reset Input
Many μP-based products require manual reset capability, allowing the operator, a test technician, or
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external logic circuitry to initiate a reset. A logic low on M R asserts reset. Reset remains asserted
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while M R is low, and for the Reset Active Timeout Period (t_RP) after M R returns high. This input
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has an internal 20kΩ pull-up resistor, so it can be left open if it is not used. M R can be driven with
TTL or CMOS-logic levels, or with open-drain/collector outputs. Connect a normally open
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momentary switch from M R to GND to create a manual-reset function; external debounce
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circuitry is not required. If M R is driven from long cables or if the device is used in a noisy
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environment, connecting a 0.1μF capacitor from M R to ground provides additional noise
immunity.
Reset Threshold Accuracy
The UM805/811/812 are ideal for systems using a 5V±5% or 3V±5% power supply with ICs
specified for 5V±10% or 3V±10%, respectively. They are designed to meet worst-case
specifications over temperature. The reset is guaranteed to assert after the power supply falls out of
regulation, but before power drops below the minimum specified operating voltage range for the
system ICs. The thresholds are pre-trimmed and exhibit tight distribution, reducing the range over
which an undesirable reset may occur.
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http://www.union-ic.com Rev.01 Jan.2015
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