MAX691ACSLASHD datasheet(13/17 Pages) MAXIM

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old and power is lost at VCC, the SuperCap on VBATT

discharges through VCC until VBATT reaches the reset

threshold; then the battery-backup mode is initiated and

the current through VCC goes to zero.

Using Separate Power Supplies

for VBATT and VCC

If using separate power supplies for VCC and VBATT,

VBATT must be less than 0.3V above VCC when VCC is

above the reset threshold. As described in the previous

section, if VBATT exceeds this limit and power is lost at

VCC, current flows continuously from VBATT to VCC via

the VBATT-to-VOUT diode and the VOUT-to-VCC switch

until the circuit is broken (Figure 8).

Alternate Chip-Enable Gating

Using memory devices with both CE and CE inputs allows

the CE loop to be bypassed. To do this, connect CE IN to

ground, pull up CE OUT to VOUT, and connect CE OUT to

the CE input of each memory device (Figure 10). The CE

input of each part then connects directly to the chip-select

logic, which does not have to be gated.

MAX691A

MAX693A

MAX800L

MAX800M

VOUT

GND

CE IN

CE OUT

*MAXIMUM Rp VALUE DEPENDS ON

THE NUMBER OF RAMS.

MINIMUM Rp VALUE IS 1kΩ.

ACTIVE-HIGH

CE LINES

FROM LOGIC

RAM 1

RAM 2

RAM 3

RAM 4

Rp*

MAX691A

MAX693A

MAX800L

MAX800M

VCC

GND

PFI

*OPTIONAL

VIN

+5V

C1*

TO µP

PFO

VTRIP = 1.25 R1 + R2

VH = 1.25/

R2 I I R3

VL - 1.25 + 5 - 1.25 = 1.25

R1 + R2 I I R3

PFO

VTRIP

VIN

MAX691A

MAX693A

MAX800L

MAX800M

VCC

GND

PFI

+5V

PFO

NOTE: VTRIP IS NEGATIVE.

VTRIP

V-

5 - 1.25 = 1.25 - VTRIP

V-

Figure10. Alternate CE Gating

Figure 11. Adding Hysteresis to the Power-Fail Comparator

Figure 12. Monitoring a Negative Voltage

MAX691A/MAX693A/

MAX800L/MAX800M

Microprocessor Supervisory Circuits

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