S1220PBID Datasheet, PDF (7/18 Pages) ON Semiconductor – Universal Voltage Monitors

MC34161, MC33161, NCV33161
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 14.
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 MW 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 V_CC 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 latchup 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 15 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 6. The input circuitry forms a tristate switch with
thresholds at 0.63 V and V_ref + 0.23 V. The mode select input
current is 10 mA when connected to the reference output, and
42 mA when connected to a V_CC of 5.0 V, refer to Figure 7.
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 11 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 V_CC of
1.0 V. These characteristics are important in undervoltage
sensing applications where the output must stay in a low
state as V_CC approaches ground. Figure 5 shows the Output
Voltage versus Supply Voltage in an undervoltage sensing
application. Note that as V_CC drops below the programmed
4.5 V trip point, the output stays in a well defined active low
state until V_CC 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 16 has
the LED ‘ON’ when input voltage V_S is above trip voltage
V₂, for overvoltage detection. The dashed line output
connection has the LED ‘ON’ when V_S is below trip voltage
V₂, for undervoltage detection.
http://onsemi.com
7