MAX803 Series, NCP803 Series
www.onsemi.com
7
APPLICATIONS INFORMATION
V
CC
Transient Rejection
The MAX803/NCP803 series provides accurate V
CC
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 13 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 5.0 ms or less will not cause a
reset pulse. Transient immunity can be improved by adding
a capacitor in close proximity to the V
CC
pin of the
MAX803.
Figure 13. Maximum Transient Duration vs.
Overdrive for Glitch Rejection at 25°C
Duration
V
TH
Overdrive
CC
10
250
200
11060
MAXIMUM TRANSIENT DURATION (
m
50
RESET COMPARATOR OVERDRIVE (mV)
0
41
150
100
V
TH
= 1.2 V
160 210 260 310 360
V
TH
= 4.63 V
RESET Signal Integrity During Power−Down
The MAX803/NCP803 RESET output is valid to V
CC
=
1.0 V. Below this voltage the output becomes an “open
circuit” and does not sink current. This means CMOS logic
inputs to the Microprocessor will be floating at an
undetermined voltage. Most digital systems are completely
shutdown well above this voltage. However, in situations
where RESET
must be maintained valid to V
CC
= 0 V, since
the NCP803/MAX803 has Open−Drain and active−low
output, it typically uses a pullup resistor. With this device,
RESET will most likely not maintain an active condition,
but will drift to a non−active level due to the pullup resistor
and the reduced sinking capability of the open−drain device.
Therefore, this device is not recommended for applications
where the RESET
pin is required to be valid down to
V
CC
= 0 V.
V
CC
V
CC
R1
100 k*
MAX809
NCP803
GND
Figure 14. RESET Signal Integrity
RESET
*Assume High−Z Reset Input to Microprocessor
MAX803 RESET Output Allows Use With Two Power
Supplies
In numerous applications the pullup resistor place 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 microprocessor. However, if the
NCP803/MAX803’s supply goes blew 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 V
CC
level that depends
on the R
pullup
value and the voltage which is connected.
V
CC
V
CC
2
Microprocessor
RESET
MAX803
NCP803
GND GND
R
pullup
Figure 15. MAX803 RESET Output with Two Supplies
RESET
V
CC
V
CC
1
