ADM690AARN データシートの表示（PDF） - Analog Devices

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ADM690AARN

Microprocessor Supervisory Circuits

Analog Devices 

ADM690A/ADM692A/ADM802L/M/ADM805L/M

VBATT

VCC

BATTERY

SWITCHOVER

VOUT

4.65V*

RESET

GENERATOR

RESET

(RESET)

WATCHDOG

INPUT (WDI)

POWER FAIL

INPUT (PFI)

WATCHDOG

TRANSITION DETECTOR

(1.6s)

1.25V

ADM690A

ADM692A

ADM802L

ADM802M

ADM805L

ADM805M

POWER FAIL

OUTPUT (PFO)

*4.4V FOR ADM692A/ADM802M/ADM805M

( ) = ADM805L/M ONLY

Figure 7. Functional Block Diagram

POWER FAIL RESET, RESET

RESET is an active low output which 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 (ADM690A/

ADM802L/ADM805L or 4.4 V ADM692A/ADM802M/

ADM805M.

On power-up RESET will remain low for 200 ms after VCC rises

above the 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.

The guaranteed minimum and maximum thresholds are as follows:

ADM690A/ADM802L/ADM805L: 4.5 V and 4.75 V

ADM692A: 4.25 V and 4.5 V.

ADM802L: 4.55 V and 4.7 V

ADM802M: 4.3 V and 4.45 V

The ADM805L and ADM805M contain an active high reset

output. This is the complement of RESET and is intended for

processors requiring an active high RESET signal.

The guaranteed minimum and maximum thresholds for the

ADM805 are:

ADM805L: 4.5 V and 4.75 V

ADM805M: 4.25 V and 4.5 V.

Watchdog Timer RESET, RESET

The watchdog timer circuit monitors the activity of the micro-

processor in order to check that it is not stalled in an indefinite

loop. An output line on the processor is used to toggle the

Watchdog Input (WDI) line. If this line is not toggled within

1.6 seconds, a RESET pulse is generated. The watchdog

timeout period restarts with each transition on the WDI pin. To

ensure that the watchdog timer does not time out, either a

high-to-low or low-to-high transition on the WDI pin must

occur at or less than the minimum timeout period. If WDI

remains permanently either high or low, reset pulses will be

issued after each timeout period (1.6 s). The watchdog monitor

can be deactivated by floating the Watchdog Input (WDI) or by

connecting it to midsupply.

+5V

VCC

0V

VOUT

3.0V

0V

+5V

RESET

0V

+5V

RESET

0V

3.0V

+5V

PFO

0V

tRS

VBATT = PFI = 3.0V

Figure 8. Timing Diagram

BATTERY SWITCHOVER SECTION

During normal operation with VCC higher than the reset

threshold, VCC is internally switched to VOUT via an internal

PMOS transistor switch. This switch has a typical on-resistance

of less than 1 Ω and can supply up to 100 mA at the VOUT

terminal. Once VCC falls below the reset threshold, the higher of

VCC or VBATT is switched to VOUT. This means that VBATT

connects to VOUT only when VCC is below the reset threshold

and VBATT is greater than VCC.

VOUT is normally used to drive a RAM memory bank which

may require instantaneous 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 capacitance value of 0.1 µF or greater may be used.

A 9 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 0.05 µA.

Typically 3 V batteries are used as the backup supply. High

value capacitors, either standard electrolytic or the farad size

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

back up. 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 back up battery by compensating for

its self discharge current. Also note that this current poses no

problem when lithium batteries are used for back up 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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