MAX691AD Ver la hoja de datos (PDF) - Maxim Integrated

Número de pieza

componentes Descripción

Fabricante

MAX691AD

Microprocessor Supervisory Circuits

Maxim Integrated 

Microprocessor Supervisory Circuits

Battery-Backup Input

The Battery-Backup Input (VBATT) is similar to the VCC

input except the PMOS switch and parallel diode are

much smaller. Accordingly, the on-resistances of the

diode and the switch are each approximately 10Ω.

Continuous current should be limited to 25mA and

peak currents (only during power-up) limited to 250mA.

The reverse leakage of this input is less than 1µA over

temperature and supply voltage (Figure 8).

Output Supply Voltage

The Output Supply Voltage (VOUT) pin is internally con-

nected to the substrate of the IC and supplies current

to the external system and internal circuitry. All open-

circuit outputs will, for example, assume the VOUT volt-

age in their high states rather than the VCC voltage. At

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

cally be 200mV below VCC. Decouple this terminal with

a 0.1µF capacitor.

__________Applications Information

The MAX691A/MAX693A/MAX800L/MAX800M are not

short-circuit protected. Shorting VOUT to ground, other

than power-up transients such as charging a decou-

pling 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, insure that

these leads 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 35µA 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

1µA max.

Using SuperCap™ or MaxCap™ with the

MAX691A/MAX693A/MAX800L/MAX800M

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 9).

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 5.1V typ. Leakage current through

the SuperCap charging diode and the internal power

diode eventually discharges the SuperCap to VCC. Also, if

VCC and VBATT start from 0.1V above the reset threshold

and power is lost at VCC, the SuperCap on VBATT dis-

charges through VCC until VBATT reaches the reset

threshold; then the battery-backup mode is initiated and

the current through VCC goes to zero.

VBATT

VCC

MAX691A

MAX693A

MAX800L

MAX800M

VOUT

0.1µF

Figure 8. VCC and VBATT to VOUT Switch

+5V

3

1N4148

VCC

0.47F*

* MaxCap

1

VBATT

2

VOUT

MAX691A

MAX693A

MAX800L

MAX800M

GND

4

Figure 9. SuperCap or MaxCap on VBATT

12 ______________________________________________________________________________________

Share Link: