AS1925-BSTT datasheet(8/12 Pages) AMSCO

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AS1925/AS1926

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

9 Application Information

Figure 9. Typical Application Diagram

Fast Transient Immunity

The AS1925/AS1926 are virtually immune to fast, falling VCC transients. Figure 8 on page 6 shows typical transient

duration versus reset comparator overdrive, for which the devices do not generate a reset pulse. The graph was gen-

erated using a falling pulse applied to VCC, starting 0.1V above the actual reset threshold and ending below it by the

magnitude indicated (reset comparator overdrive). The graph indicates the maximum pulse width that a falling VCC

transient can have without causing a reset pulse. As the magnitude of the transient goes further below the reset

threshold, the maximum allowable pulse width decreases.

Note: For additional transient immunity, place a 0.1µF bypass capacitor as close as is practical to pin VCC.

Ensuring a Valid Reset Output Down to 0V

If VCC falls below 0.55V, the AS1925 push/pull RESETN output will no longer sink current and becomes an open cir-

cuit, which means that high-impedance CMOS-logic inputs connected to RESETN can drift to undetermined voltages.

This is not relevant for most applications since most microprocessors and other circuits do not operate with a supply

voltage < 0.55V.

In systems where RESETN must be valid down to 0V, adding a pulldown resistor to RESETN (see Figure 10) causes

any stray leakage currents to flow to ground, holding RESETN low. The value of the pulldown resistor is not critical; a

200k

Ω resistor is large enough not to load RESETN and small enough to pull RESETN to ground.

Figure 10. Reset Valid to VCC = 0V

Note: A 200k

Ω pullup resistor to VCC is also recommended if the push/pull RESET is required to remain valid for VCC

0.75V.

AS1926

External

Reset

CPU

RESETN

VCC

I/O

Supply

0.9V

GND

DC/DC

Converter

+3.3V

GND

VCC

RESETN

MRN

AS1925

RPULLDOWN

GND

VCC

RESETN

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