MAX791CUE 查看數據表（PDF） - Maxim Integrated

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MAX791CUE

Microprocessor Supervisory Circuit

Maxim Integrated 

Microprocessor Supervisory Circuit

+5V

1N4148

0.47F

3

VCC

1

VBATT

2

VOUT

MAX791

GND

4

Figure 12. SuperCap or MaxCap on VBATT

Applications Information

The MAX791 is not short-circuit protected. Shorting

VOUT to ground, other than power-up transients such

as charging a decoupling capacitor, destroys the

device.

All open-circuit outputs swing between VOUT and GND

rather than VCC and GND.

If long leads connect to the chip inputs, ensure that

these lines are free from ringing and other conditions

that would forward bias the chip’s protection diodes.

There are three distinct modes of operation:

1) Normal operating mode with all circuitry powered

up. Typical supply current from VCC is 60µA, while

only leakage currents flow from the battery.

2) Battery-backup mode where VCC is typically within

0.7V below VBATT. All circuitry is powered up and

the supply current from the battery is typically less

than 60µA.

3) Battery-backup mode where VCC is less than

VBATT by at least 0.7V. VBATT supply current is

less than 1µA max.

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 typ. Leakage

current through the SuperCap charging diode and

MAX791 internal power diode eventually discharges 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 so 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 current

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 10kΩ to prevent it

from loading down the PFO pin. Capacitor C1 adds

additional noise rejection.

14 ______________________________________________________________________________________

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