ADM705 データシートの表示（PDF） - Analog Devices

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ADM705

Low Cost Microprocessor Supervisory Circuits

Analog Devices 

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 simply 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

1.25V –

PFO

+

TO

PFI

MICROPROCESSOR

R2

ADM705/ADM706/ NMI

ADM707/ADM708

R3

5V

PFO

0V

0V VL

VH

VIN

Figure 16. Adding Hysteresis to the Power-Fail Comparator

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 used to 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

⎟⎠⎞

ADM705/ADM706/ADM707/ADM708

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 be connected

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. G | Page 9 of 12

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