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ADM8693ANZ

Microprocessor Supervisory Circuits

Analog Devices 

ADM8690/ADM8691/ADM8692/ADM8693/ADM8695

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 14. Battery Switchover Schematic

During normal operation, with VCC higher than VBATT, VCC

is internally switched to VOUT through an internal PMOS tran-

sistor 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, requiring

instantaneous currents of greater than 100 mA. If this is the

case, a bypass capacitor should be connected to VOUT. The

capacitor provides the peak current transients to the RAM.

A capacitance value of 0.1 μF or greater can 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 can be connected in parallel

with the internal transistor. The BATT ON output (ADM8691/

ADM8693/ADM8695) can directly drive the base of the

external transistor.

A 7 Ω MOSFET switch connects the VBATT input to VOUT during

battery backup. This MOSFET has very low input-to-output

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 typically 0.4 μA.

The ADM8690/ADM8691/ADM8695 operate with battery

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

ADM8693 operate 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 maximum) 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

compensating for its self-discharge current. Also note that this

current poses no problem when lithium batteries are used for

backup because 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.

POWER-FAIL RESET OUTPUT

RESET is an active low output that provides a RESET signal to

the microprocessor whenever VCC is at an invalid level. When

VCC falls below the reset threshold, the RESET output is forced

low. The nominal reset voltage threshold is 4.65 V (ADM8690/

ADM8691/ADM8695) or 4.4 V (ADM8692/ADM8693).

VCC

V2

V1

V2

V1

RESET

t1

t1

LOW LINE

t1 = RESET TIME

V1 = RESET VOLTAGE THRESHOLD LOW

V2 = RESET VOLTAGE THRESHOLD HIGH

HYSTERESIS = V2–V1

Figure 15. Power-Fail Reset Timing

On power-up, RESET remains low for 50 ms (200 ms for

ADM8695) after VCC rises above the appropriate reset threshold.

This allows time for the power supply and microprocessor to

stabilize. On power-down, the RESET output remains low with

VCC as low as 1 V. This ensures that the microprocessor is held

in a stable shutdown condition.

This RESET active time is adjustable on the ADM8691/ADM8693/

ADM8695 by using an external oscillator or by connecting an

external capacitor to the OSC IN pin. Refer to Table 5 and

Figure 17, Figure 18, Figure 19, and Figure 20.

The guaranteed minimum and maximum thresholds of the

ADM8690/ADM8691/ADM8695 are 4.5 V and 4.73 V, and the

guaranteed thresholds of the ADM8692/ADM8693 are 4.25 V and

4.48 V. The ADM8690/ADM8691/ADM8695 are, therefore,

compatible with 5 V supplies with a +10%, −5% tolerance and

the ADM8692/ADM8693 are compatible with 5 V ± 10%

supplies. The reset threshold comparator has approximately

50 mV of hysteresis. The response time of the reset voltage

comparator is less than1 μs. If glitches are present on the VCC line

that could cause spurious reset pulses, VCC should be decoupled

close to the device.

Rev. B | Page 10 of 20

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