ADM708ARMZ-REEL(2016) View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADM708ARMZ-REEL
(Rev.:2016)

Low Cost Microprocessor Supervisory Circuits

Analog Devices 

Data Sheet

Adding Hysteresis to the Power Fail Comparator

For increased noise immunity, hysteresis can be added to the

power fail comparator. Because the comparator circuit is

noninverting, hysteresis can be added by connecting a resistor

between the PFO output and the PFI input as shown in Figure 16.

7V TO 15V

INPUT

POWER

5V

ADP3367

R1

VCC

TO

MICROPROCESSOR

1.25V –

PFO

NMI

+

PFI

RESET

R2

ADM705/ADM706/

TO

MICROPROCESSOR

ADM707/ADM708

RESET

R3

5V

PFO

0V

0V VL

VH

VIN

Figure 16. Adding Hysteresis to the Power Fail Comparator

ADM705/ADM706/ADM707/ADM708

When PFO is low, Resistor R3 sinks current from the summing

junction at the PFI pin. When PFO is high, Resistor R3 sources

current into the PFI summing junction. This results in differing

trip levels for the comparator. Further noise immunity can be

achieved by connecting a capacitor between PFI and GND. The

equations calculate the hysteresis are as follows:

VH

 1.251  

R2

R2





R3

R3

R1

VL

 1.25



R1



1.25

R2



VCC  1.25

R3





VMID

 1.25



R1  R2

R2





VALID RESET BELOW 1 V VCC

The ADM705/ADM706/ADM707/ADM708 are guaranteed to

provide a valid reset level with VCC as low as 1 V (see the Typical

Performance Characteristics section). As VCC drops below 1 V,

the internal transistor does not have sufficient drive to hold the

voltage RESET at 0 V. A pull-down resistor can connect externally,

as shown in Figure 17, to hold the line low if required.

ADM705/ADM706/

ADM707/ADM708

RESET

GND

R1

Figure 17. RESET Valid Below 1 V

Rev. H | Page 9 of 12

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