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MAX697CWE

Microprocessor Supervisory Circuits

Maxim Integrated 

MAX696/MAX697

Microprocessor Supervisory Circuits

MAX697

The MAX697 is nearly identical to the MAX696. The MAX697

lacks the battery-backup feature, so it does not have the

VBATT, VOUT, or BATT ON pins. This allows the MAX697

to consume less than 250 microamperes, and it allows the

inclusion of RAM write-protection pins. See Figure 2.

Detailed Description

Battery Switchover and VOUT (MAX696)

Battery Switchover and VOUT (MAX696) The battery-

switchover circuit compares VCC to the VBATT input, and

connects VOUT to whichever is higher. Switchover occurs

when VCC is 50mV greater than VBATT as VCC falls, and

when VCC is 70mV more than VBATT as VCC rises (see

Figure 3). The switchover comparator has 20mV of hys-

teresis to prevent repeated, rapid switching if VCC falls

very slowly or remains nearly equal to the battery voltage.

When VCC is higher than VBATT, VCC is internally

switched to VOUT with a low-saturation pnp transistor.

VOUT has 50mA output current capability. Use an external

pnp pass transistor in parallel with the internal transistor

if the output current requirement at VOUT exceeds 50mA

or if a lower VCC - VOUT voltage differential is desired.

The BATT ON output can directly drive the base of the

external transistor.

It should be noted that the MAX696 need only supply

the average current drawn by the CMOS RAM if there is

adequate filtering. Many RAM data sheets specify a 75mA

maximum supply current, but this peak current spike lasts

only 100ns. A 0.1μF bypass capacitor at VOUT supplies

the high instantaneous current, while VOUT need only

supply the average load current, which is much less. A

capacitance of 0.1μF or greater must be connected to the

VOUT terminal to ensure stability.

A 200Ω MOSFET connects the VBATT input to VOUT

during battery backup. This MOSFET has very low input-

to-output differential (dropout voltage) at the low current

levels required for battery backup of CMOS RAM or other

low-power CMOS circuitry. When VCC equals VBATT, the

supply current is typically 12μA. When VCC is between 0V

and (VBATT - 700mV), the typical supply current is only

600nA (typ), 1μA (max).

VCC

+5V

VCC

BASE DRIVE

P

p-CHANNEL

MOSFET

VOUT

0.1µF

TO CMOS

RAM AND

REAL-TIME

CLOCK

VCC IN

3V

BATTERY

INPUT

VBATT

100mV

+

-

700mV

INTERNAL

+

LOW IQ MODE SELECT SHUTDOWN

-

SIGNAL WHEN

VBATT > VCC + 0.7V

Figure 3. MAX696 Battery-Switchover Block Diagram

www.maximintegrated.com

BATT ON

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