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MAX703

Low-Cost Microprocessor Supervisory Circuits with Battery Backup

Maxim Integrated 

MAX703/MAX704

Low-Cost Microprocessor Supervisory

Circuits with Battery Backup

Detailed Description

RESET Output

A μP’s reset input starts the μP in a known state.

Whenever the μP is in an unknown state, it should be held

in reset. The MAX703/MAX704 assert reset when VCC is

low, preventing code-execution errors during power-up,

power-down, or brownout conditions.

When VBATT is 2V or more, RESET is always valid,

irrespective of VCC. On power-up, as VCC rises, RESET

remains low. When VCC exceeds the reset threshold, an

internal timer holds RESET low for a time equal to the

reset pulse width (typically 200ms); after this interval,

RESET goes high (Figure 2). If a power-fail or brown-

out condition occurs (i.e., VCC drops below the reset

threshold), RESET is asserted. As long as VCC remains

below the reset threshold, the internal timer is continually

restarted, causing the RESET output to remain low. Thus,

a brownout condition that interrupts a previously initiated

reset pulse causes an additional 200ms delay from the

end of the last interruption.

Power-Fail Comparator

The PFI input is compared to an internal reference. If PFI

is less than 1.25V, PFO goes low. The power-fail com-

parator can be used as an undervoltage detector to signal

a failing power supply. In the Typical Operating Circuit,

an external voltage-divider at PFI is used to monitor the

unregulated DC voltage from which the regulated +5V

supply is derived.

The voltage-divider can be chosen so the voltage at PFI falls

below 1.25V just before the +5V regulator drops out. PFO is

then used as an interrupt to prepare the μP for power-down.

To conserve power, the power-fail comparator is turned off

and PFO is forced low when the MAX703/MAX704 enter

battery-backup mode.

Backup-Battery Switchover

In the event of a brownout or power failure, it may be nec-

essary to preserve the contents of RAM. With a backup

battery installed at VBATT, the MAX703/MAX704 auto-

matically switch RAM to backup power when VCC fails.

As long as VCC exceeds the reset threshold, VCC con-

nects to VOUT through a 5Ω p-channel MOSFET power

switch. Once VCC falls below the reset threshold, RESET

goes low and VCC or VBATT (whichever is higher) switch-

es to VOUT. Note that VBATT switches to VOUT through

an 80Ω switch only if VCC is below the resetthreshold volt-

age and VBATT is greater than VCC. When VCC exceeds

the reset threshold, it is connected to the MAX703/

MAX704 substrate, regardless of the voltage applied to

VBATT (Figure 3). During this time, diode D1 (between

VBATT and the substrate) conducts current from VBATT to

VCC if VBATT ≥ (VCC + 0.6V).

When the battery-backup mode is activated, VBATT con-

nects to VOUT. In this mode, the substrate connects to

VBATT and internal circuitry is powered from the battery

(Figure 3). Table 1 shows the status of the MAX703/

MAX704 inputs and outputs in battery-backup mode.

When VCC is below, but within, 1V of VBATT, the internal

switchover comparator draws about 30μA. Once VCC

Table 1. Input and Output Status in

Battery-Backup Mode

SIGNAL

VCC

VOUT

VBATT

RESET

PFI

PFO

MR

STATUS

Disconnected from VOUT.

Connected to VBATT through an internal 80Ω

p-channel MOSFET switch.

Connected to VOUT. Supply current is < 1µA

when VCC < (VBATT - 1V).

Logic-low.

Power-fail comparator is disabled.

Logic-low.

Disabled.

VBATT

VCC

MAX703

S1

S2

MAX704

S3

S4

D1

D2

SUBSTRATE

D3

VOUT

CONDITION

VCC > Reset Threshold

VCC < Reset Threshold and

VCC > VBATT

VCC < Reset Threshold and

VCC < VBATT

S1/S2

Open

Open

S1/S2

Closed

Closed

Closed Open

Figure 3. Battery-Switchover Block Diagram

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