MCP809, MCP810

SNVS165A – JUNE 2001 – REVISED MAY 2013

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APPLICATION INFORMATION

Benefits of Precision Reset Thresholds

A microprocessor supply supervisor must provide a reset output within a predictable range of the supply voltage.

A common threshold range is between 5% and 10% below the nominal supply voltage. The 4.63V and 3.08V

options of the MCP809/810 use highly accurate circuitry to ensure that the reset threshold occurs only within this

range (for 5V and 3.3V supplies). The other voltage options have the same tight tolerance to ensure a reset

signal for other narrow monitor ranges. See Table 1 for examples of how the standard reset thresholds apply to

3V, 3.3V, and 5V nominal supply voltages.

Table 1. Reset Thresholds Related to Common Supply Voltages

Reset Threshold

3.0V

3.3V

5.0V

4.63 ± 3%

90 - 95%

4.38 ± 3%

85 - 90%

4.00 ± 3%

78 - 82%

3.08 ± 3%

90 - 95%

2.93 ± 3%

86 - 90%

2.63 ± 3%

85 - 90%

77 - 81%

Ensuring a Valid Reset Output Down to VCC = 0V

When VCC falls below 1V, the MCP809 RESET output no longer sinks current. A high-impedance CMOS logic

input connected to RESET can therefore drift to undetermined voltages. To prevent this situation, a 100k

Ω

resistor should be connected from the RESET output to ground, as shown in Figure 9.

A 100k

Ω pull-up resistor to VCC is also recommended for the MCP810, if RESET is required to remain valid for

VCC < 1V.

Figure 9. RESET Valid to VCC = Ground Circuit

Negative-Going VCC Transients

The MCP809/810 are relatively immune to short negative-going transients or glitches on VCC. Figure 10 shows

the maximum pulse width a negative-going VCC transient can have without causing a reset pulse. In general, as

the magnitude of the transient increases, going further below the threshold, the maximum allowable pulse width

decreases. Typically, for the 4.63V and 4.38V version of the MCP809/810, a VCC transient that goes 100mV

below the reset threshold and lasts 20µs or less will not cause a reset pulse. A 0.1 µF bypass capacitor mounted

as close as possible to the VCC pin will provide additional transient rejection.

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