MAX803 Series, NCP803 Series

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7

APPLICATIONS INFORMATION

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

MAX803.

Figure 13. Maximum Transient Duration vs.

Overdrive for Glitch Rejection at 25°C

Duration

Overdrive

10

250

200

110

60

50

RESET COMPARATOR OVERDRIVE (mV)

0

410

150

100

160

210

260

310

360

RESET Signal Integrity During Power−Down

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

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

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

Microprocessor

RESET

MAX803

NCP803

GND

GND

Figure 15. MAX803 RESET Output with Two Supplies

RESET