MAX791CSLASHD datasheet(15/20 Pages) MAXIM

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Using SuperCaps or MaxCaps

with the MAX791

VBATT has the same operating voltage range as VCC,

and the battery-switchover threshold voltages are typi-

cally ±30mV centered at VBATT, allowing use of a

SuperCap and a simple charging circuit as a backup

source (Figure 12).

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

above VCC, current flows to VOUT and VCC from VBATT

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

For example, with a SuperCap connected to VBATT

and through a diode to VCC, if VCC quickly changes

from 5.4V to 4.9V, the capacitor discharges through

VOUT and VCC until VBATT reaches 5.3V typical.

Leakage current through the SuperCap charging diode

and MAX791 internal power diode eventually dis-

charges the SuperCap to VCC. Also, if VCC and VBATT

start from 0.5V above the reset threshold and power is

lost at VCC, the SuperCap on VBATT discharges

through VCC until VBATT reaches the reset threshold;

the MAX791 then switches to battery-backup mode and

the current through VCC goes to zero (Figure 10).

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 10).

Alternative Chip-Enable Gating

Using memory devices with CE and CE inputs allows

the MAX791 CE loop to be bypassed. To do this, con-

nect CE IN to ground, pull up CE OUT to VOUT, and

connect CE OUT to the CE input of each memory

device (Figure 13). The CE input of each part then con-

nects directly to the chip-select logic, which does not

have to be gated by the MAX791.

Adding Hysteresis to the

Power-Fail Comparator

Hysteresis adds a noise margin to the power-fail com-

parator and prevents repeated triggering of PFO when

VIN is near the power-fail comparator trip point. Figure

14 shows how to add hysteresis to the power-fail com-

parator. Select the ratio of R1 and R2 such that PFI

sees 1.25V when VIN falls to the desired trip point

(VTRIP). Resistor R3 adds hysteresis. It will typically be

an order of magnitude greater than R1 or R2. The cur-

rent through R1 and R2 should be at least 1µA to

ensure that the 25nA (max) PFI input current does not

shift the trip point. R3 should be larger than 10k

Ω to

prevent it from loading down the PFO pin. Capacitor C1

adds additional noise rejection.

Microprocessor Supervisory Circuit

______________________________________________________________________________________

MAX791

VCC

GND

PFI

*OPTIONAL

VIN

+5V

C1*

µP

PFO

VTRIP = 1.25

R1 + R2

VH = 1.25 /

|| R3

VL - 1.25

5 - 1.25

1.25

R1 + R2

|| R3

PFO

+5V

VTRIP

VIN

Figure 14. Adding Hysteresis to the Power-Fail Comparator

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