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Fabricante

ADM8694AN

Microprocessor Supervisory Circuits

Analog Devices 

ADM8690–ADM8695

PIN CONFIGURATIONS

VOUT 1 ADM8690 8 VBATT

VCC 2 ADM8692 7 RESET

GND 3 ADM8694 6 WDI

TOP VIEW

PFI 4 (Not to Scale) 5 PFO

VBATT 1

16 RESET

VOUT 2

15 RESET

ADM8691

VCC 3 ADM8693 14 WDO

GND 4 ADM8695 13 CEIN

BATT ON

5

TOP VIEW 12

(Not to Scale)

CEOUT

LOW LINE 6

11 WDI

OSC IN 7

10 PFO

OSC SEL 8

9 PFI

Part

Number

ADM8690

ADM8691

ADM8692

ADM8693

ADM8694

ADM8695

Nominal Reset

Time

50 ms

50 ms or ADJ

50 ms

50 ms or ADJ

200 ms

200 ms or ADJ

PRODUCT SELECTION GUIDE

Nominal VCC

Reset Threshold

Nominal Watchdog Battery Backup

Timeout Period

Switching

4.65 V

4.65 V

4.4 V

4.4 V

4.65 V

4.65 V

1.6 s

Yes

100 ms, 1.6 s, ADJ Yes

1.6 s

Yes

100 ms, 1.6 s, ADJ Yes

1.6 s

Yes

100 ms, 1.6 s, ADJ Yes

Base Drive

Ext PNP

No

Yes

No

Yes

No

Yes

Chip Enable

Signals

No

Yes

No

Yes

No

Yes

CIRCUIT INFORMATION

Battery Switchover Section

The battery switchover circuit compares VCC to the VBATT

input, and connects VOUT to whichever is higher. Switchover

occurs when VCC is 50 mV higher than VBATT as VCC falls, and

when VCC is 70 mV greater than VBATT as VCC rises. This

20 mV of hysteresis prevents repeated rapid switching if VCC

falls very slowly or remains nearly equal to the battery voltage.

VCC

VBATT

VOUT

GATE DRIVE

100

mV

700

mV

INTERNAL

SHUTDOWN SIGNAL

WHEN

VBATT > (VCC + 0.7V)

BATT ON

(ADM8690,

ADM8695)

Figure 1. Battery Switchover Schematic

During normal operation, with VCC higher than VBATT, VCC is

internally switched to VOUT via an internal PMOS transistor

switch. This switch has a typical on-resistance of 0.7 Ω and can

supply up to 100 mA at the VOUT terminal. VOUT is normally

used to drive a RAM memory bank which may require instanta-

neous currents of greater than 100 mA. If this is the case then a

bypass capacitor should be connected to VOUT. The capacitor

will provide the peak current transients to the RAM. A capaci-

tance value of 0.1 µF or greater may be used.

If the continuous output current requirement 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 (ADM8691/

ADM8693/ADM8695) can directly drive the base of the exter-

nal transistor.

A 7 Ω MOSFET switch connects the VBATT input to VOUT dur-

ing battery backup. This MOSFET has very low input-to-out-

put differential (dropout voltage) at the low current levels

required for battery back up of CMOS RAM or other low power

CMOS circuitry. The supply current in battery back up is typi-

cally 0.4 µA.

The ADM8690/ADM8691/ADM8694/ADM8695 operates with

battery voltages from 2.0 V to 4.25 V, and the ADM8692/

ADM8693 operates with battery voltages from 2.0 V to 4.0 V.

High value capacitors, either standard electrolytic or the farad

size double layer capacitors, can also be used for short-term

memory backup. A small charging current of typically 10 nA

(0.1 µA max) flows out of the VBATT terminal. This current is

useful for maintaining rechargeable batteries in a fully charged

condition. This extends the life of the backup battery by com-

pensating for its self discharge current. Also note that this cur-

rent poses no problem when lithium batteries are used for

backup since the maximum charging current (0.1 µA) is safe for

even the smallest lithium cells.

If the battery switchover section is not used, VBATT should be

connected to GND and VOUT should be connected to VCC.

REV. 0

–5–

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