ADM8696ARWZ View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADM8696ARWZ

Microprocessor Supervisory Circuits

Analog Devices 

APPLICATIONS INFORMATION

Increasing the Drive Current (ADM8696)

If the continuous output current requirements at VOUT exceeds

100 mA or if a lower VCC–VOUT voltage differential is desired,

an external PNP pass transistor may be connected in parallel

with the internal transistor. The BATT ON output (ADM8696)

can directly drive the base of the external transistor.

+5V

INPUT

POWER

BATTERY

0.1µF

PNP

TRANSISTOR

0.1µF

VCC

BATT

ON

VBATT

ADM8696

VOUT

Figure 14. Increasing the Drive Current

Using a Rechargeable Battery for Backup (ADM8696)

If a capacitor or a rechargeable battery is used for backup, the

charging resistor should be connected to VOUT since this elimi-

nates the discharge path that would exist during power-down if

the resistor is connected to VCC.

+5V

INPUT

POWER

0.1µF

RECHARGABLE

BATTERY

I = VOUT – VBATT

R

R

0.1µF

VCC

VOUT

VBATT

ADM8696

Figure 15. Rechargeable Battery

Adding Hysteresis to the Power Fail Comparator

For increased noise immunity, hysteresis may be added to the

power fail comparator. Since the comparator circuit is nonin-

verting, hysteresis can be added by connecting a resistor be-

tween the PFO output and the PFI input as shown in Fig-

ure 16. When PFO is low, resistor R3 sinks current from the

summing junction at the PFI pin. When PFO is high, the series

combination of R3 and R4 source current into the PFI summing

junction. This results in differing trip levels for the comparator.

Alternate Watchdog Input Drive Circuits

The watchdog feature can be enabled and disabled under pro-

gram control by driving WDI with a three-state buffer (Figure

17a). When three-stated, the WDI input will float, thereby dis-

abling the watchdog timer.

ADM8696/ADM8697

This circuit is not entirely foolproof and it is possible a software

fault could erroneously three-state the buffer. This would pre-

vent the ADM869x from detecting that the microprocessor is no

longer operating correctly. In most cases, a better method is to

+7V TO +15V

INPUT

POWER

R1

R2

+5V

7805

VCC

1.3V

PFO

PFI

ADM869x

R3

R4

TO

µP NMI

( ) VH = 1.3V

1+

–R–1–

R2

+

–RR–13–

( ) VL = 1.3V

1+ –R–1– – –R–1––(5–V––––1–.–3–V–)–

R2 1.3V (R3 + R4)

( ) ASSUMING R4 < < R3 THEN

HYSTERESIS VH – VL = 5V

–RR–12–

Figure 16. Adding Hysteresis to the Power Fail Comparator

extend the watchdog period rather than disabling the watchdog.

This may be done under program control using the circuit

shown in Figure 17b. When the control input is high, the OSC

SEL pin is low and the watchdog timeout is set by the external

capacitor. A 0.01 µF capacitor sets a watchdog timeout delay of

100 s. When the control input is low, the OSC SEL pin is

driven high, selecting the internal oscillator. The 100 ms or the

1.6 s period is chosen, depending on which diode in Fig-

ure 17b is used. With D1 inserted, the internal timeout is set at

100 ms while with D2 inserted the timeout is set at 1.6 s.

WATCHDOG

STROBE

CONTROL

INPUT

WDI

ADM869x

Figure 17a. Programming the Watchdog Input

CONTROL

INPUT*

D1

OSC SEL

D2

ADM869x

OSC IN

*LOW = INTERNAL TIMEOUT

HIGH = EXTERNAL TIMEOUT

Figure 17b. Programming the Watchdog Input

REV. A

–9–

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