AS1925 Просмотр технического описания (PDF) - austriamicrosystems AG

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AS1925

Ultra-Low Voltage Microprocessor Supervisory Circuit with Manual Reset

austriamicrosystems AG 

AS1925/AS1926

Data Sheet - Application Information

9 Application Information

Figure 9. Typical Application Diagram

+3.3V

DC/DC

Converter

External

Reset

0.9V

5

VCC

4

MRN

AS1926

GND 2

1

RESETN

VCC I/O

Supply

CPU

RESETN

GND

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

200kΩ resistor is large enough not to load RESETN and small enough to pull RESETN to ground.

Figure 10. Reset Valid to VCC = 0V

RPULLDOWN

5

VCC

1 AS1925

RESETN

GND 2

Note: A 200kΩ pullup resistor to VCC is also recommended if the push/pull RESET is required to remain valid for VCC

0.75V.

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