2013 Microchip Technology Inc. DS20005172B-page 9
RE46C800
3.0 DEVICE DESCRIPTION
3.1 Introduction
The RE46C800 provides the necessary analog
functions to build a microcontroller-based CO or toxic
gas detector. This includes an op amp and voltage
reference for the electrochemical sensor, a voltage
regulator for the microcontroller, an LED driver, a horn
driver, a detector interconnect function, a boost regula-
tor for 3V operation, a power management system that
allows operation from 3V, 9V or AC derived power. The
power management system provides the capability for
AC power with battery backup. The RE46C800
provides a simple means for the microcontroller to
control the operation of the CO detector and provide
the necessary signaling functions during an alarm
condition.
3.2 CO Sensor Circuit
The RE46C800 provides a low offset op amp and
reference voltage, V
REF
, for a two terminal
electrochemical CO or toxic gas sensor. The unity gain
stable op amp provides rail-to-rail inputs and output.
The op amp output is monitored by the microcontroller
to determine the CO concentration. This uncommitted
op amp can be used for other purposes such as
temperature sensing.
3.3 Power Management System
The power management system allows the RE46C800
to be powered from a 3V or 9V battery or AC power. AC
power is supplied as a DC voltage derived from an AC
power supply. This DC voltage is diode connected to
the V
BST
pin of the RE46C800. AC supplied power and
a 9V battery can both be diode connected to the V
BST
pin.
For low-voltage systems the battery is connected to the
V
DD
pin. When only a low-voltage battery is available,
the internal circuitry is powered from V
DD
. When a 9V
battery or AC power is available, the internal circuitry is
powered from V
REG
, which is a regulated 3.3V. The
selection of the power source for the internal circuitry is
controlled with the ACDET pin when the 9VDET pin is
low.
In low-voltage systems that are also AC powered, the
boost regulator will turn on if voltage of the AC supplied
power drops below the specified boost regulator
voltage. This can cause the low-voltage battery to
discharge more rapidly than expected.
The 9VDET pin will disable the boost regulator if
9VDET is high. For a low-voltage system, the 9VDET
pin should be connected to V
SS
which will enable the
boost regulator.
Table 3-1 shows the truth table for the power
management system.
3.4 Boost Regulator
The boost regulator only operates in low-voltage
applications. The boost regulator is a fixed off time
boost regulator with peak current limiting. In low-boost
operation the peak current is nominally 0.6A. In high-
boost operation the peak current is nominally 1.2A. The
boost regulator normally operates in Low-Boost mode,
which provides a nominal 4V output voltage on the
V
BST
pin. In High-Boost mode, the boost regulator
provides a nominal 9.8V on the V
BST
pin. The boost
regulator can be placed in High-Boost mode with
HORNEN, LEDEN, or IODIR and IO2 both asserted
high.
The brown-out threshold voltage is the V
BST
voltage at
which the voltage regulator and the horn will be
disabled. When the V
BST
voltage falls below the brown-
out threshold voltage of 3.6V, V
REG
will be disabled and
pulled to V
SS
with a nominal 40 mA current. When the
boost voltage rises above the brown-out threshold
voltage, V
REG
is enabled.
3.5 Voltage Regulator
The voltage regulator provides a nominal 3.3V output
at the V
REG
pin and is intended to power a microcon-
troller. In normal operation, the regulator will source
current up to 20 mA, but the current sinking capability
is typically under 1 µA. The voltage regulator is pow-
ered from the V
BST
pin. In low-voltage applications the
regulator is powered by the boost regulator and the
regulator load current is part of the boost regulator load
current. An overvoltage clamp is intended to limit the
voltage at V
REG
if it is pulled up by an external source
to greater than 4V. When the boost regulator experi-
ences a brown-out condition, the voltage regulator will
be disabled and the V
REG
output will be pulled to V
SS
.
TABLE 3-1: POWER MANAGEMENT
SYSTEM
9VDET ACDET
Internal
Supply
Boost Regulator
00V
DD
Enabled
01V
REG
Enabled
10V
REG
Disabled
11V
REG
Disabled
