MAX694CSLASHD datasheet(8/18 Pages) MAXIM

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Detailed Description

Battery-Switchover and VOUT

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

Figure 4). The switchover comparator has 20mV of hyster-

esis 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 via a low saturation PNP transis-

tor. VOUT has 50mA output current capability. Use an

external PNP pass transistor in parallel with internal tran-

sistor if the output current requirement at VOUT exceeds

50mA or if a lower VCC-VOUT voltage differential is

desired. The BATT ON output (MAX691/MAX693/MAX695

only) can directly drive the base of the external transistor.

It should be noted that the MAX690–MAX695 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 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 typical, 1µA maximum.

The MAX690/MAX691/MAX694/MAX695 operate with

battery voltages from 2.0V to 4.25V while MAX692/

MAX693 operate with battery voltages from 2.0V to

4.0V. High value capacitors can also be used for short-

term memory backup. External circuitry is required to

ensure that the capacitor voltage does not rise above

the reset threshold, and that the charging resistor does

not discharge the capacitor when in backup mode. The

MAX691A and the MAX791 provide solutions requiring

fewer external components.

A small charging current of typically 10nA (0.1µA max)

flows out of the VBATT terminal. This current varies with the

amount of current that is drawn from VOUT but its polarity

is such that the backup battery is always slightly charged,

and is never discharged while VCC is in its operating volt-

age range. This extends the shelf life of the backup battery

by compensating for its self-discharge current. Also note

that this current poses no problem when lithium batteries

are used for backup since the maximum charging current

(0.1µA) is safe for even the smallest lithium cells.

If the battery-switchover section is not used, connect

VBATT to GND and connect VOUT to VCC. Table 2 shows

the state of the inputs and output in the low power battery

backup mode.

Figure 3. MAX691/MAX693/MAX695 Block Diagram

RESET GENERATOR

TIMEBASE FOR RESET

AND

WATCHDOG

WATCHDOG TRANSITION

DETECTOR

WATCHDOG

TIMER

1.3V

4.65V

VBATT

BATT ON

VCC

CHIP-ENABLE INPUT

OSC IN

*4.4V (MAX693)

OSC SEL

WATCHDOG INPUT

POWER FAIL

INPUT

VOUT

CHIP ENABLE OUTPUT

LOW LINE

RESET

GROUND

RESET

WATCHDOG OUTPUT

POWER FAIL OUTPUT

MAX690–MAX695

Microprocessor Supervisory Circuits

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