TABLE 1. Monitored Tolerance Table

Reset

Threshold

Supply Voltage

2.5V

3.3V

5.0V

90.8-94.4%

91.8-95.2%

90.8-94.8%

Ensuring a Valid Reset Output Down to V

When V

unable to sink the rated current. A high-impedance CMOS

logic input connected to RESET can therefore drift to unde-

termined voltages. To prevent this situation, a 100k

Ω resistor

should be connected from the RESET output to ground, as

shown in

Figure 1.

A 100k

Ω pull-up resistor to V

LM3723, if RESET is required to remain valid for V

Open Drain Output (LM3724)

An open drain output allows easy paralleling of multiple

microprocessor reset circuits without requiring additional

logic gates. Open drain outputs also allow interfacing de-

vices of differing logic levels or families, since the output

pull-up resistor may be connected to any supply voltage up

to 5.5V, regardless of LM3724 V

The pull up resistor is calculated so that maximum current

flow into RESET is less than 10 mA when activated. The

resistor must be small enough so that the leakage current of

all connected devices does not create an excessive voltage

drop when the output is not activated. A resistor value of 100

k

Ω will generally suffice.

Negative-Going V

The LM3722/LM3723/LM3724 are relatively immune to short

negative-going transients or glitches on V

the maximum pulse width a negative-going V

can have without causing a reset pulse. In general, as the

magnitude of the transient increases, going further below the

threshold, the maximum allowable pulse width decreases.

Typically, a V

threshold and lasts 40 µs or less will not cause a reset pulse.

A 0.1 µF bypass capacitor mounted as close as possible to

the V

Interfacing to µPs with Bidirectional Reset Pins

Microprocessors with bidirectional reset pins, such as the

Motorola 68HC11 series, can be connected to the LM3722

RESET output. To ensure a correct output on the LM3722

even when the microprocessor reset pin is in the opposite

state, connect a 4.7k

Ω resistor between the LM3722 RESET

output and the µP reset pin, as shown in

Figure 3. Buffer the

LM3722 RESET output to other system components.

20014410

FIGURE 1. Circuit for RESET Valid from V

20014414

FIGURE 2. Maximum Transient Duration without

Causing a Reset Pulse vs. Reset Comparator

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