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NCP803 Datasheet(PDF) 4 Page - ON Semiconductor

Part No. NCP803
Description  Very Low Supply Current 3-Pin Microprocessor Reset Monitor
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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NCP803 Datasheet(HTML) 4 Page - ON Semiconductor

   
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NCP803
http://onsemi.com
4
APPLICATIONS INFORMATION
VCC Transient Rejection
The NCP803 provides accurate VCC monitoring and reset
timing during power-up, power-down, and brownout/sag
conditions, and rejects negative-going transients (glitches)
on the power supply line. Figure 2 shows the maximum
transient duration vs. maximum negative excursion
(overdrive) for glitch rejection. Any combination of
duration and overdrive which lies under the curve will not
generate a reset signal. Combinations above the curve are
detected as a brownout or power-down. Typically, transient
that goes 100 mV below the reset threshold and lasts 5
µs or
less will not cause a reset pulse. Transient immunity can be
improved by adding a capacitor in close proximity to the VCC
pin.
Figure 2. Maximum Transient Duration vs. Overdrive
for Glitch Rejection at 25
°C
Duration
VTH
Overdrive
VCC
10
250
200
40
30
50
300
RESET COMPARATOR OVERDRIVE (mV)
0
60
150
100
20
50
70
80
90 100
VTH = 3.08 V
VTH = 1.6 V
VTH = 4.9 V
Processors With Bidirectional I/O Pins
Some
µP’s (such as Motorola 68HC11) have
bi-directional reset pins which interface easily with the
Open Drain RESET output of the NCP803. As shown in
Figure 3, one can connect directly to the RESET output of
the NCP803 to the RESET pin of the
µP. The pull-up resistor
avoids an undetermined voltage of the RESET pin.
Figure 3. Interfacing to Bidirectional Reset I/O
VCC
VCC
VCC
RESET
RESET
NCP803
GND
GND
µP
0
0
Rpull- up
C = 100 nF
NCP803 RESET Output Allows Use With Two Power
Supplies
In numerous applications the pull-up resistor placed on
the RESET output is connected to the supply voltage
monitored by the IC. Nevertheless, a different supply
voltage can also power this output and so level-shift from
the monitored supply to reset the
µP. However, if the
NCP803’s supply goes below 1 V, the RESET output ability
to sink current will decrease and the result is a high state on
the pin even though the supply’s IC is under the threshold
level. This occurs at a VCC level that depends on the Rpull- up
value and the voltage to which it is connected.
Figure 4. RESET Output with Two Power Supplies
VCC
VCC
RESET
RESET
NCP803
GND
GND
µP
0
0
Rpull- up
C = 100 nF
+3.3 V
+5.0 V


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