MAX817MESA datasheet(13/16 Pages) MAXIM

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+5V Microprocessor Supervisory Circuits

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Watchdog Input Current

The MAX817/MAX818 WDI inputs are internally driven

through a buffer and series resistor from the watchdog

counter (Figure 1). When WDI is left unconnected, the

watchdog timer is serviced within the watchdog timeout

period by a low-high-low pulse from the counter chain.

For minimum watchdog input current (minimum overall

power consumption), leave WDI low for the majority of the

watchdog timeout period, pulsing it low-high-low once

within 7/8 of the watchdog timeout period to reset the

watchdog timer. If instead WDI is externally driven high for

the majority of the timeout period, up to 150µA can flow

into WDI.

Using a SuperCap™ as a

Backup Power Source

SuperCaps are capacitors with extremely high capaci-

tance values (on the order of 0.47F) for their size. Since

BATT has the same operating voltage range as VCC, and

the battery switchover threshold voltages are typically

±30mV centered at VBATT, a SuperCap and simple

charging circuit can be used as a backup power source.

Figure 11 shows a SuperCap used as a backup source.

If VCC is above the reset threshold and VBATT is 0.5V

above VCC, current flows to OUT and VCC from BATT

until the voltage at BATT is less than 0.5V above VCC.

For example, if a SuperCap is connected to BATT

through a diode to VCC, and VCC quickly changes from

5.4V to 4.9V, the capacitor discharges through OUT

and VCC until VBATT reaches 5.1V typical. Leakage cur-

rent through the SuperCap charging diode and the

internal power diode eventually discharges the

SuperCap to VCC. Also, if VCC and VBATT start from

0.1V above the reset threshold and power is lost at

VCC, the SuperCap on BATT discharges through VCC

until VBATT reaches the reset threshold. Battery-backup

mode is then initiated and the current through VCC

goes to zero.

Operation Without a

Backup Power Source

The MAX817/MAX818/MAX819 were designed for bat-

tery-backed applications. If a backup battery is not

used, connect VCC to OUT, and connect BATT to

ground.

Replacing the Backup Battery

The backup power source can be removed while VCC

remains valid, without danger of triggering a reset

pulse, if BATT is decoupled with a 0.1µF capacitor to

ground. As long as VCC stays above the reset thresh-

old, battery-backup mode cannot be entered.

Adding Hysteresis to the Power-Fail

Comparator (MAX817/MAX819)

The power-fail comparator has a typical input hystere-

sis of 4mV. This is sufficient for most applications where

a power-supply line is being monitored through an

external voltage divider (see

Monitoring an Additional

Supply).

For additional noise margin, connect a resistor between

PFO and PFI, as shown in Figure 12. Select the ratio of

R1 and R2 such that PFI sees VPFT when VIN falls to the

Figure 11. Using a SuperCap™ as a Backup Power Source

with a +5V ±10% Supply

SuperCap is a trademark of Baknor Industries.

BATT

VCC

OUT

RESET

GND

TO STATIC RAM

µP

0.1F

MAX817

MAX818

MAX819

+5V

100k

Figure 12. Adding Hysteresis to the Power-Fail Comparator

VCC

GND

µP

PFI

PFO

*OPTIONAL

C1*

VIN

+5V

R1 + R2

VH = 1.25V

VTRIP = 1.25V

MAX817

MAX819

PFO

+5V

VTRIP

VIN

(

)

VL - 1.25

- 1.25

1.25

(

)

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