February 2007

Rev 1

1/14

AN2504

Application note

Using STMicroelectronics Voltage Detectors

1

Introduction

Voltage detectors are designed to monitor the system supply voltage and assert the output

signal, OUT, every time it goes below a defined voltage threshold.

The major advantages of these circuits are low current consumption and a precise

voltage detectors are also laser programmed to the desired voltage threshold over the range

of 1.6V to 6.0V in 100mV steps and they have a good transient immunity (see Figure 5).

Figure 1.

N-Channel Open Drain Output Block Diagram

Note that voltage detector output state simply indicates if the supply voltage is above or

below a specific threshold, which can be used for early power fail warning.

However microprocessors' inputs require some minimum input pulse width to register the

change of signal on the logical input. In this case we recommend using reset circuits, which

guarantee the minimum pulse width known as reset time-out period. Reset circuits have

some other advantages compared to voltage detectors (e.g. specification across

temperature, better voltage threshold accuracy, and lower temperature coefficient). On the

other hand voltage detectors are usually able to operate at higher voltages, have

lower

current consumption, greater hysteresis and usually a lower price.

The qualities mentioned above cause voltage detectors to be preferentially used in the

following applications:

•

Back-up Supply Switching

•

Portable/Battery-

Powered Electronics

•

Cell Phones.

AI10482b

VCC

R1

R2

R3

VSS

+

–

VREF

VOUT

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