概要

The MAX811/MAX812 are low-power microprocessor (μP) supervisory circuits used to monitor power supplies in μP and digital systems. They provide excellent circuit reliability and low cost by eliminating external components and adjustments when used with 5Vpowered or 3V-powered circuits. The MAX811/MAX812 also provide a debounced manual reset input.

These devices perform a single function: They assert a reset signal whenever the VCC supply voltage falls below a preset threshold, keeping it asserted for at least 140ms after VCC has risen above the reset threshold. The only difference between the two devices is that the MAX811 has an active-low RESET output (which is guaranteed to be in the correct state for VCC down to 1V), while the MAX812 has an active-high RESET output. The reset comparator is designed to ignore fast transients on VCC. Reset thresholds are available for operation with a variety of supply voltages.

Low supply current makes the MAX811/MAX812 ideal for use in portable equipment. The devices come in a 4-pin SOT143 package.

 

アプリケーション

Computers

Controllers

Intelligent Instruments

Critical μP and μC Power Monitoring

Portable/Battery-Powered Equipment

 

メリットと特徴

Integrated Voltage Monitor Increases System Robustness with Added Manual Reset

Precision Monitoring of 3V, 3.3V, and 5V Power-Supply Voltages

140ms Min Power-On-Reset Pulse Width

RESET Output (MAX811), RESET Output (MAX812)

Guaranteed Over Temperature

Guaranteed RESET Valid to VCC = 1V (MAX811)

Power-Supply Transient Immunity

Saves Board Space

No External Components

4-Pin SOT143 Package

Low Power Consumption Simplifies Power-Supply Requirements

6μA 消費電流

 

詳細

Reset Output

A microprocessor’s (μP’s) reset input starts the μP in a known state. These μP supervisory circuits assert reset to prevent code execution errors during power-up, powerdown, or brownout conditions.

RESET is guaranteed to be a logic low for VCC > 1V. Once VCC exceeds the reset threshold, an internal timer keeps RESET low for the reset timeout period; after this interval, RESET goes high.

If a brownout condition occurs (VCC dips below the reset threshold), RESET goes low. Any time the reset threshold, the internal timer resets to zero, and RESET goes low. The internal timer starts after VCC returns above the reset threshold, and RESET remains low for the reset timeout period.

The manual reset input (MR) can also initiate a reset. See the Manual Reset Input section.

The MAX812 has an active-high RESET output that is the inverse of the MAX811’s RESET output.

 

Reset Threshold Accuracy

The MAX811/MAX812 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.

 

アプリケーション情報

Negative-Going VCC Transients

In addition to issuing a reset to the μP during power-up, power-down, and brownout conditions, the MAX811/MAX812 are relatively immune to short duration negativegoing VCC transients (glitches).Shows typical transient durations vs. reset comparator overdrive, for which the MAX811/MAX812 do not generate a reset pulse. This 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, a VCC transient that goes 125mV below the reset threshold and lasts 40μs or less (MAX81_L/M) or 20μs or less (MAX81_T/S/R) will not cause a reset pulse to be issued. A 0.1μF capacitor mounted as close as possible to VCC provides additional transient immunity.