AX6901/2/3/4
3-Pin Microprocessor Reset Circuits
GENERAL DESCRIPTION
TheAX6901/2/3/4 is used for microprocessor (µP) supervisory circuits to monitor the
power supplies in µP and digital systems. They provide excellent circuit reliability and low
cost by eliminating external components and adjustments when used with +5V, +3.3V, +3.0V,
+2.5V powered circuits.
These circuits perform a single function: they assert a reset signal whenever the VCC
supply voltage declines below a preset threshold, keeping it asserted for at least 200ms
after VCC has risen above the reset threshold. Reset thresholds suitable for operation with
a variety of supply voltages are available. TheAX6901/2/3/4 has push pull outputs.
TheAX6901/3 has an active low
RESET
output, while theAX6902/4 has an active high
RESET output. The reset comparator is designed to ignore fast transients on VCC, and the
outputs are guaranteed to be in the correct logic state for VCC down to 1.0V. Low supply
current makes theAX6901/2/3/4 ideal for use in portable equipment. TheAX6901/2/3/4 is
available in a 3-pin SOT23 package.
FEATURES
- Precision Monitoring of +2.5,+3V, +3.3V, and +5V Power-Supply Voltages
- Fully Specified Over Temperature
- Available in Three Output Configurations
-
-
-
-
-
-
-
-
Push-Pull
RESET
Low Output(AX6901/3)
Push-Pull RESET High Output(AX6902/4
200ms (Typ.)min Power-On Reset Pulse Width
25µA Supply Current
Guaranteed Reset Valid to V
CC
= +1.0V
Power Supply Transient Immunity
No External Components
Available in the 3-Pin Pb-Free SOT-23 Package
BLOCK DIAGRAM
VCC
P-MOS
-
+
Delay
Circuit
N-MOS
RESET
/ RESET
VREF
GND
1/9
Axelite Confidential Materials, do not copy or distribute without written consent.
Rev.2.0 Dec. 25, 2015
AX6901/2/3/4
FUNCTION DESCRIPTIONS
A microprocessor’s (µP’s) reset input starts the µP in a known state. The
AX6901/2/3/4 asserts reset to prevent code-execution errors during power-up, power-down,
or brownout conditions. They assert a reset signal whenever the VCC supply voltage
declines below a preset threshold, keeping it asserted for at least 200ms after VCC has
risen above the reset threshold. The AX6901/2/3/4 has a push-pull output stage.
APPLICATION INFORMATION
Negative-Going VCC Transients
In addition to issuing a reset to the µP during power-up, power-down, and brownout
conditions, the AX6901/2/3/4 is relatively immune to short-duration negative-going VCC
transients (glitches).
The AX6901/2/3/4 does not generate a reset pulse. The graph was generated using a
negative going pulse applied to VCC, starting 0.5V above the actual reset threshold and
ending below it by the magnitude indicated (reset comparator overdrive). The graph
indicates the maximum pulse width a negative going VCC transient can have without
causing a reset pulse. 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 50mV below the reset threshold A 0.1µF bypass capacitor mounted as close as
possible to the VCC pin provides additional transient immunity.
Ensuring a Valid Reset Output Down to VCC = 0
RESET
is guaranteed to be a logic low for VCC > 1.0V. 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 VCC goes below 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.
When VCC falls below 1.0V, theAX6901/3 RESET output no longer sinks current—it
becomes an open circuit. Therefore, high-impedance CMOS logic inputs connected
to
RESET
can drift to undetermined voltages.
This presents no problem in most applications since most µP and other circuitry is
inoperative with VCC below 1.0V. However, in applications where
RESET
must be valid
down to 0V, adding a pull down resistor to
RESET
causes any stray leakage currents to flow
to ground, holding
RESET
low. R1’s value is not critical; 100k is large enough not to load
RESET
and small enough to pull
RESET
to ground. For theAX6902/4 if RESET is
required to remain valid for VCC < 1.0V.
5/9
Axelite Confidential Materials, do not copy or distribute without written consent.
Rev.2.0 Dec. 25, 2015
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