MAX807LEUE datasheet(13/16 Pages) MAXIM

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Battery-Backup Mode

Battery backup preserves the contents of RAM in the

event of a brownout or power failure. With a backup

battery installed at BATT, the MAX807 automatically

switches RAM to backup power when VCC falls. Two

conditions are required for switchover to battery-back-

up mode: 1) VCC must be below the reset threshold; 2)

VCC must be below VBATT. Table 1 lists the status of

inputs and outputs during battery-backup mode.

Backup-Battery Input

The BATT input is similar to VCC, except the PMOS

switch is much smaller. This input is designed to con-

duct up to 20mA to OUT during battery backup. The

on-resistance of the PMOS switch is approximately

Ω. Figure 12 shows the two series pass elements

between the BATT input and OUT that facilitate UL

approval. VBATT can exceed VCC during normal opera-

tion without causing a reset.

Output Supply Voltage

The output supply (OUT) transfers power from VCC or

BATT to the µP, RAM, and other external circuitry. At

the maximum source current of 250mA, VOUT will typi-

cally be 260mV below VCC. Decouple this terminal with

a 0.1µF capacitor.

BATT ON Output

The battery on (BATT ON) output indicates the status of

the internal battery switchover comparator, which con-

trols the internal VCC and BATT switches. For VCC

greater than VBATT (ignoring the small hysteresis

effect), BATT ON typically sinks 3.2mA at 0.4V. In bat-

tery-backup mode, this output sources approximately

5mA. Use BATT ON to indicate battery switchover sta-

tus, or to supply gate or base drive for an external pass

transistor for higher current applications (see the

Typical Operating Circuit).

Full-Featured µP Supervisory Circuit with

±1.5% Reset Accuracy

______________________________________________________________________________________

Figure 12. VCC and BATT-to-OUT Switch

MAX807

OUT

VCC

BATT

0.1

µF

CONTROL

CIRCUITRY

Table 1. Input and Output Status in Battery-Backup Mode

The power-fail comparator remains active in battery-backup mode for VCC

≥ 4V. Below 4V, PFO is forced low.

PFO

WDI is ignored and goes high impedance

WDI

Battery switchover comparator monitors VCC for active switchover.

VCC

MR is ignored

Logic high; the open-circuit output voltage is equal to VCC.

RESET

Logic low

LOW LINE

Ground—0V reference for all signals

GND

Logic high. The open-circuit output voltage is equal to VCC.

WDO

High impedance

CE IN

Logic high. The open-circuit output voltage is equal to VBATT.

CE OUT

Supply current is 1µA maximum for VBATT

≤ 2.8V.

BATT

OUT is connected to BATT through two internal PMOS switches in series.

OUT

The power-fail comparator remains active in battery-backup mode for VCC

≥ 4V.

PFI

FUNCTION

NAME

Logic high when VBATT exceeds 2.285V. Valid for VCC

≥ 4V. Below 4V, BATT OK is forced low.

BATT OK

Logic high. The open-circuit output voltage is equal to VBATT.

BATT ON

Logic low

RESET

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