ADM8690ANZ データシートの表示（PDF） - Analog Devices

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ADM8690ANZ

Microprocessor Supervisory Circuits

Analog Devices 

Data Sheet

TYPICAL APPLICATIONS

ADM8690 APPLICATIONS

Figure 30 shows the ADM8690 in a typical power monitoring,

battery backup application. VOUT powers the CMOS RAM.

Under normal operating conditions with VCC present, VOUT

is internally connected to VCC. If a power failure occurs, VCC

decays and VOUT is switched to VBATT, thereby maintaining

power for the CMOS RAM. A RESET pulse is also generated

when VCC falls below 4.65 V for the ADM8690. RESET remains

low for 50 ms after VCC returns to 5 V.

5V

R1

VCC

POWER

PFI

VOUT

CMOS RAM

POWER

0.1µF

R2

ADM8690

MICROPROCESSOR

SYSTEM

+

BATTERY

VBATT

RESET

PFO

GND WDI

RESET

NMI

I/O LINE

Figure 30. ADM8690 Typical Application, Circuit A

The watchdog timer input (WDI) monitors an input/output

line from the microprocessor system. This line must be toggled

once every 1.6 sec to verify correct software execution. Failure

to toggle the line indicates that the microprocessor system is

not correctly executing its program and may be tied up in an

endless loop. If this happens, a reset pulse is generated to

initialize the microprocessor.

If the watchdog timer is not needed, the WDI input should be

left floating.

The power-fail input, PFI, monitors the input power supply via

a resistive divider network. The voltage on the PFI input is

compared with a precision 1.3 V internal reference. If the input

voltage drops below 1.3 V, a power-fail output (PFO) signal is

generated. This signal warns of an impending power failure and

can be used to interrupt the processor so that the system can be

shut down in an orderly fashion. The resistors in the sensing

network are ratioed to give the desired power-fail threshold

voltage (VT).

VT = (1.3 R1/R2) + 1.3 V

R1/R2 = (VT/1.3) − 1

ADM8690/ADM8691/ADM8695

Figure 31 shows a similar application, but in this case the PFI

input monitors the unregulated input to the 7805 voltage regu-

lator. This circuit provides an earlier warning of an impending

power failure. It is useful with processors that operate at low

speeds or where there are a significant number of housekeeping

tasks to be completed before the power is lost.

INPUT

POWER

V > 8V

R1

R2

5V

7805

VCC

PFI

VOUT

ADM8690

0.1µF

0.1µF

POWER

CMOS RAM

POWER

MICROPROCESSOR

SYSTEM

+

BATTERY

VBATT

RESET

PFO

GND WDI

RESET

NMI

I/O LINE

Figure 31. ADM8690 Typical Application, Circuit B

ADM8691/ADM8695 APPLICATIONS

Figure 32 shows a typical connection for the ADM8691/ADM8695.

CMOS RAM is powered from VOUT. When 5 V power is present,

this voltage is routed to VOUT. If VCC fails, VBATT is routed to VOUT.

VOUT can supply up to 100 mA from VCC, but if more current is

required, an external PNP transistor can be added. When VCC is

higher than VBATT, the BATT ON output goes low, providing up

to 35 mA of base drive for the external transistor. A 0.1 μF capac-

itor is connected to VOUT to supply the transient currents for

CMOS RAM. When VCC is lower than VBATT, an internal 20 Ω

MOSFET connects the backup battery to VOUT.

INPUT POWER

5V

0.1µF

0.1µF

3V

BATTERY

R1

R2

NC

VCC

VBATT

BATT VOUT

ON

CEOUT

ADM8691/ CEIN

PFI ADM8695

GND

WD I

OSC IN

PFO

OSC SEL

RESET

LOW LINE WDO

CMOS

RAM

ADDRESS

DECODE

RESET

A0 TO A15

I/O LINE

NMI

RESET

0.1µF MICROPROCESSOR

SYSTEM

SYSTEM STATUS

INDICATORS

Figure 32. ADM8691/ADM8695 Typical Application

Rev. C | Page 17 of 24

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