MAX791CSLASHD datasheet(7/20 Pages) MAXIM

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Microprocessor Supervisory Circuit

_______________________________________________________________________________________

______________________________________________________________Pin Description

NAME

FUNCTION

VBATT

2VOUT

Output Supply Voltage. VOUT connects to VCC when VCC is greater than VBATT and VCC is above the reset

threshold. When VCC falls below VBATT and VCC is below the reset threshold, VOUT connects to VBATT.

Connect a 0.1µF capacitor from VOUT to GND.

3VCC

Input Supply Voltage—+5V input

GND

BATT ON

–

—

–

PFI

SWT

–

—

–

—

–

WDI

–

—

–

OUT

Chip-Enable Output.

–

—

–

OUT goes low only when

–

—

–

IN is low and VCC is above the reset threshold. If

–

—

–

IN is

low when reset is asserted,

–

—

–

OUT will stay low for 15µs or until

–

—

–

IN goes high, whichever occurs first.

–

—

–

Backup-Battery Input. Connect to external battery or capacitor and charging circuit.

Ground. 0V reference for all signals

Battery On Output. Goes high when VOUT switches to VBATT. Goes low when VOUT switches to VCC. Connect

the base of a PNP through a current-limiting resistor to BATT ON for VOUT current requirements greater than

250mA.

Power-Fail Output. This is the output of the power-fail comparator.

–

—

–

goes low when PFI is less than 1.25V.

This is an uncommitted comparator, and has no effect on any other internal circuitry.

Power-Fail Input. This is the noninverting input to the power-fail comparator. When PFI is less than 1.25V,

–

—

–

goes low. Connect PFI to GND or VOUT when not used.

Set Watchdog-Timeout Input. Connect this input to VOUT to select the default 1.6sec watchdog timeout period.

Connect a capacitor between this input and GND to select another watchdog-timeout period. Watchdog-timeout

period = 2.1 x (capacitor value in nF) ms.

–

—

–

—

–

—

–

Manual-Reset Input. This input can be tied to an external momentary pushbutton switch, or to a logic gate out-

put.

–

—

–

remains low as long as

–

—

–

is held low and for 200ms after

–

—

–

returns high.

–

—

–

Output goes low when VCC falls to 150mV above the reset threshold. The output can be used to gen-

erate an NMI if the unregulated supply is inaccessible.

Watchdog Input. WDI is a three-level input. If WDI remains either high or low for longer than the watchdog time-

out period,

–

—

–

goes low.

–

—

–

remains low until the next transition at WDI. Leaving WDI unconnected disables

the watchdog function. WDI connects to an internal voltage divider between VOUT and GND, which sets it to mid-

supply when left unconnected.

Chip-Enable Input. The input to chip-enable gating circuit. Connect to GND or VOUT if not used.

Watchdog Output.

–

—

–

goes low if WDI remains either high or low longer than the watchdog timeout period.

–

—

–

returns high on the next transition at WDI.

–

—

–

remains high if WDI is unconnected.

–

—

–

is also high

when

–

—

–

is asserted.

–

—

–

Output goes low whenever VCC falls below the reset threshold.

–

—

–

will remain low for typically 200ms

after VCC crosses the reset threshold on power-up.

Watchdog-Pulse Output. Upon the absence of a transition at WDI,

–

—

–

will pulse low for a minimum of 1ms.

–

—

–

precedes

–

—

–

by 70ns.

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