NCP803

http://onsemi.com

4

APPLICATIONS INFORMATION

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

pin.

Figure 2. Maximum Transient Duration vs. Overdrive

for Glitch Rejection at 25

°C

Duration

Overdrive

10

250

200

40

30

50

300

RESET COMPARATOR OVERDRIVE (mV)

0

60

150

100

20

50

70

80

90 100

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

RESET

RESET

NCP803

GND

GND

µP

0

0

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

value and the voltage to which it is connected.

Figure 4. RESET Output with Two Power Supplies

RESET

RESET

NCP803

GND

GND

µP

0

0

C = 100 nF

+3.3 V

+5.0 V