MAX793_EPE datasheet(14/20 Pages) MAXIM

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3.0V/3.3V Adjustable Microprocessor

Supervisory Circuits

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The power-fail comparator turns off and PFO goes low

when VCC falls below VSW on power-down. During the

first half of the reset timeout period (tRP), PFO is forced

high, irrespective of VPFI. At the beginning of the sec-

ond half of tRP, the power-fail comparator is enabled

and PFO follows PFI. If the comparator is unused, con-

nect PFI to VCC and leave PFO unconnected. PFO can

be connected to MR so that a low voltage on PFI gener-

ates a reset (Figure 12b). In this configuration, when

the monitored voltage causes PFI to fall below VPFT,

PFO pulls MR low, causing a reset to be asserted.

Reset remains asserted as long as PFO holds MR low,

and for 200ms after PFO pulls MR high when the moni-

tored supply is above the programmed threshold.

Backup-Battery Switchover

In the event of a brownout or power failure, it may be

necessary to preserve the contents of RAM. With a

backup battery installed at BATT, the devices automati-

cally switch RAM to backup power when VCC falls. In

order to allow the backup battery (e.g., a 3.6V lithium

cell) to have a higher voltage than VCC, this family of µP

supervisors (designed for 3.3V and 3V systems) does

not always connect BATT to OUT when VBATT is

greater than VCC. BATT connects to OUT (through a

140

Ω switch) either when VCC falls below VSW and

VBATT is greater than VCC, or when VCC falls below

1.75V (typ) regardless of the BATT voltage.

Switchover at VSW ensures that battery-backup mode is

entered before VOUT gets too close to the 2.0V mini-

mum required to reliably retain data in most CMOS

RAM, (switchover at higher VCC voltages would

decrease backup-battery life). When VCC recovers,

switchover is deferred either until VCC crosses VBATT if

VBATT is below VRST, or when VCC rises above the

reset threshold (VRST) if VBATT is above VRST. This

power-up switchover technique prevents VCC from

charging the backup battery through OUT when using

an external transistor driven by BATT ON. OUT con-

nects to VCC through a 4

Ω (max) PMOS power switch

when VCC crosses the reset threshold (Figure 13).

BATT ON (MAX793/MAX794)

BATT ON is high when OUT is connected to BATT.

Although BATT ON can be used as a logic output to

indicate the battery switchover status, it is most often

used as a gate or base drive for an external pass tran-

sistor for high-current applications (see

Driving an

External Switch with BATT ON in the Applications

Information section). When VCC exceeds VRST on

power-up, BATT ON sinks 3.2mA at 0.4V. In battery-

backup mode, this terminal sources 100µA from BATT.

MAX793

MAX794

VCC

GND

PFI

PFO

VIN

PFO

VTRIP

VPFT = 1.237V

VPFH = 10mV

WHERE

3.0V OR 3.3V

VTRIP = R2

)

(

–

VCC

(VPFT + VPFH)

VL = R2

)

(

–

VCC

(VPFT)

NOTE: VTRIP, VL ARE NEGATIVE

VCC

MAX793

MAX794

VCC

GND

PFI

PFO

VTRIP

3.0V OR 3.3V

VIN

VTRIP =

)

(

+ R2

VPFT

VH = (VPFT + VPFH)

VCC

VIN

(b)

(a)

)

(

+ R2

Figure 12. Using the Power-Fail Comparator to Monitor an Additional Power Supply: (a) VIN Is Negative, (b) VIN Is Positive

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