AP1701/2/3/4
3-Pin Microprocessor Reset Circuits
Features
- Precision Monitoring of +3V, +3.3V, and +5V
Power-Supply Voltages
- Fully Specified Over Temperature
- Available in Three Output Configurations
- Push-Pull RESET Low Output (AP1701/3)
- Push-Pull RESET High Output (AP1702/4)
- 140ms min Power-On Reset Pulse Width
- 12µA Supply Current
- Guaranteed Reset Valid to VCC = +1V
- Power Supply Transient Immunity
- No External Components
- 3-pin SOT23 package
General Description
The AP1701/2/3/4 are 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 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 140ms after VCC has risen
above the reset threshold. Reset thresholds
suitable for operation with a variety of supply
voltages are available. The AP1701/2/3/4 have
push pull outputs. The AP1701/3 have an active low
RESET output, while the AP1702/4 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 1V. Low supply current
makes the AP1701/2/3/4 ideal for use in portable
equipment. The AP1701/2/3/4 is available in a 3-pin
SOT23 package.
Applications
- Computers
- Controllers
- Intelligent Instruments
- Critical µP and µC Power Monitoring
- Portable/Battery Powered Equipment
- Automotive
Pin Assignments
(Top View)
Pin Descriptions
Name
GND
Ground
Reset output pin
L: for AP1701/3
H: for AP1702/4
Operating voltage input
Description
GND
1
AP1701
AP1702
3 V
CC
RESET
(RESET)
V
CC
RESET (RESET)
2
SOT-23
(Top View)
RESET (RESET)
1
AP1703
AP1704
2
SOT-23
3 V
CC
GND
This datasheet contains new product information. Anachip Corp. reserves the rights to modify the product specification without notice. No liability is assumed as a result of the use of
this product. No rights under any patent accompany the sale of the product.
Rev.1.0
1/8
May. 7, 2004
AP1701/2/3/4
3-Pin Microprocessor Reset Circuits
Function Description
A microprocessor’s (µP’s) reset input starts the µP
in a known state. The AP1701/2/3/4 assert 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 140ms after VCC has risen
above the reset threshold. The AP1701/2/3/4 have
a push-pull output stage.
Applications Information
Negative-Going VCC Transients
In addition to issuing a reset to the µP during
power-up, power-down, and brownout conditions,
the AP1701/2/3/4 are relatively immune to
short-duration negative-going VCC transients
(glitches).
The AP1701/2/3/4 do 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
100mV below the reset threshold and lasts 20µs or
less will not cause a reset pulse. 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 >
1V. Once VCC exceeds the reset threshold, an
When VCC falls below 1V, the AP1701/3 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 1V. 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 the AP1702/4 if RESET is required to
remain valid for VCC < 1V.
Benefits of Highly Accurate Reset Threshold
Most µP supervisor ICs have reset threshold
voltages between 5% and 10% below the value of
nominal sup-ply voltages. This ensures a reset will
not occur within 5% of the nominal supply, but will
occur when the supply is 10% below nominal.
When using ICs rated at only the nominal supply
±5%, this leaves a zone of uncertainty where the
supply is between 5% and 10% low, and where the
reset may or may not be asserted.
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.
Anachip Corp.
www.anachip.com.tw
4/8
Rev.1.0 May.7, 2004
PCB Design
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