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MC33161

Universal voltage monitor

Motorola => Freescale 

MC34161 MC33161

FUNCTIONAL DESCRIPTION

Introduction

To be competitive in today’s electronic equipment market,

new circuits must be designed to increase system reliability

with minimal incremental cost. The circuit designer can take a

significant step toward attaining these goals by implementing

economical circuitry that continuously monitors critical circuit

voltages and provides a fault signal in the event of an

out–of–tolerance condition. The MC34161, MC33161 series

are universal voltage monitors intended for use in a wide

variety of voltage sensing applications. The main objectives

of this series was to configure a device that can be used in as

many voltage sensing applications as possible while

minimizing cost. The flexibility objective is achieved by the

utilization of a unique Mode Select input that is used in

conjunction with traditional circuit building blocks. The cost

objective is achieved by processing the device on a standard

Bipolar Analog flow, and by limiting the package to eight pins.

The device consists of two comparator channels each with

hysteresis, a mode select input for channel programming, a

pinned out reference, and two open collector outputs. Each

comparator channel can be configured as either inverting or

noninverting by the Mode Select input. This allows a single

device to perform over, under, and window detection of

positive and negative voltages. A detailed description of each

section of the device is given below with the representative

block diagram shown in Figure 13.

Input Comparators

The input comparators of each channel are identical, each

having an upper threshold voltage of 1.27 V ±2.0% with 25

mV of hysteresis. The hysteresis is provided to enhance

output switching by preventing oscillations as the comparator

thresholds are crossed. The comparators have an input bias

current of 60 nA at their threshold which approximates a

21.2 MΩ resistor to ground. This high impedance minimizes

loading of the external voltage divider for well defined trip

points. For all positive voltage sensing applications, both

comparator channels are fully functional at a VCC of 2.0 V. In

order to provide enhanced device ruggedness for hostile

industrial environments, additional circuitry was designed

into the inputs to prevent device latch–up as well as to

suppress electrostatic discharges (ESD).

Reference

The 2.54 V reference is pinned out to provide a means for

the input comparators to sense negative voltages, as well as

a means to program the Mode Select input for window

detection applications. The reference is capable of sourcing

in excess of 2.0 mA output current and has built–in short

circuit protection. The output voltage has a guaranteed

tolerance of ±2.4% at room temperature.

The 2.54 V reference is derived by gaining up the internal

1.27 V reference by a factor of two. With a power supply

voltage of 4.0 V, the 2.54 V reference is in full regulation,

allowing the device to accurately sense negative voltages.

Mode Select Circuit

The key feature that allows this device to be flexible is the

Mode Select input. This input allows the user to program

each of the channels for various types of voltage sensing

applications. Figure 14 shows that the Mode Select input has

three defined states. These states determine whether

Channel 1 and/or Channel 2 operate in the inverting or

noninverting mode. The Mode Select thresholds are shown in

Figure 5. The input circuitry forms a tristate switch with

thresholds at 0.63 V and Vref + 0.23 V. The mode select input

current is 10 µA when connected to the reference output, and

42 µA when connected to a VCC of 5.0 V, refer to Figure 6.

Output Stage

The output stage uses a positive feedback base boost

circuit for enhanced sink saturation, while maintaining a

relatively low device standby current. Figure 10 shows that

the sink saturation voltage is about 0.2 V at 8.0 mA over

temperature. By combining the low output saturation

characteristics with low voltage comparator operation, this

device is capable of sensing positive voltages at a VCC of

1.0 V. These characteristics are important in undervoltage

sensing applications where the output must stay in a low

state as VCC approaches ground. Figure 4 shows the Output

Voltage versus Supply Voltage in an undervoltage sensing

application. Note that as VCC drops below the programmed

4.5 V trip point, the output stays in a well defined active low

state until VCC drops below 1.0 V.

APPLICATIONS

The following circuit figures illustrate the flexibility of this

device. Included are voltage sensing applications for over,

under, and window detectors, as well as three unique

configurations. Many of the voltage detection circuits are

shown with the open collector outputs of each channel

connected together driving a light emitting diode (LED). This

‘ORed’ connection is shown for ease of explanation and it is

only required for window detection applications. Note that

many of the voltage detection circuits are shown with a

dashed line output connection. This connection gives the

inverse function of the solid line connection. For example, the

solid line output connection of Figure 15 has the LED ‘ON’

when input voltage VS is above trip voltage V2, for

overvoltage detection. The dashed line output connection

has the LED ‘ON’ when VS is below trip voltage V2, for

undervoltage detection.

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MOTOROLA ANALOG IC DEVICE DATA

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