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ADM8690AN

Microprocessor Supervisory Circuits

Analog Devices 

Data Sheet

ADM8690/ADM8691/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

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

(ADM8691,

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 requirements at VOUT exceed

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/

ADM8695) can directly drive the base of the external transistor

(see Figure 24).

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 backup of CMOS RAM or other low power CMOS

circuitry. The supply current in battery backup is typically 0.4 μA.

The ADM8690/ADM8691/ADM8695 operate with battery

voltages from 2.0 V to 4.25 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.

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 the

ADM8695) after VCC rises above the appropriate reset thresh-

old. 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.

The RESET active time is adjustable on the ADM8691/

ADM8695 by using an external oscillator or by connecting an

external capacitor to the OSC IN pin. See Table 5 and Figure 17

through Figure 20.

The guaranteed minimum and maximum reset thresholds for

the ADM8690/ADM8691/ADM8695 are 4.5 V and 4.73 V. The

ADM8690/ADM8691/ADM8695 are, therefore, compatible with

5 V supplies with a +10%, −5% tolerance. The reset threshold

comparator typically has 40 mV of hysteresis. The response time

of the reset voltage comparator is less than 1 μs. If glitches are

present on the VCC line that could cause spurious reset pulses,

VCC should be decoupled close to the device.

In addition to RESET, the ADM8691/ADM8695 provide an

active high RESET output. This output is the complement of

RESET and is intended for processors that require an active

high reset signal.

Rev. C | Page 11 of 24

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