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MAX794
(Rev.:1996)

3.0V/3.3V Adjustable Microprocessor Supervisory Circuits

Maxim Integrated 

3.0V/3.3V Adjustable Microprocessor

Supervisory Circuits

VRST

VCC

3.3V

VSW

3.6V

3.6V

3.3V

VOUT

VBATT = 3.6V

Figure 13. Battery Switchover Timing

Table 1. Input and Output Status in

Battery-Backup Mode

PIN NAME

STATUS

OUT

VCC

BATT ON

BATT OK

PFI

PFO

MR

WDO

WDI

RESET

RESET

BATT

LOWLINE

CE IN

CE OUT

Connected to BATT through an internal

140Ω switch

Disconnected from OUT

Pulled up to BATT

Logic low

Disabled

Logic low

Disabled, but still pulled up to VCC

Logic low

Disabled

Logic low

Pulled up to VCC

Connected to OUT

Logic low

High impedance

Pulled to BATT

__________Applications Information

These µP supervisory circuits are not short-circuit pro-

tected. Shorting VOUT to ground, excluding power-up

transients such as charging a decoupling capacitor,

destroys the device. Decouple both VCC and BATT

pins to ground by placing 0.1µF ceramic capacitors as

close to the device as possible.

Driving an External Switch with BATT ON

BATT ON can be directly connected to the base of a

PNP transistor or the gate of a PMOS transistor. The

PNP connection is straightforward: connect the emitter

to VCC, the collector to OUT, and the base to BATT ON

(Figure 14a). No current-limiting resistor is required,

but a resistor connecting the base of the PNP to BATT

ON can be used to limit the current drawn from VCC,

prolonging battery life in portable equipment.

If you are using a PMOS transistor, however, it must be

connected backwards from the traditional method.

Connect the gate to BATT ON, the drain to VCC, and

the source to OUT (Figure 14b). This method orients

the body diode from VCC to OUT and prevents the

backup battery from discharging through the FET when

its gate is high. Two PMOS transistors in the Siliconix

LITTLE FOOT™ series are specified with VGS down to

-2.7V. The Si9433DY has a maximum 100mΩ drain-

source on-resistance with 2.7V of gate drive and a 2A

drain-source current. The Si9434DY specifies a 60mΩ

drain-source on-resistance with 2.7V of gate drive and

a 5.1A drain-source current.

Using a SuperCap™ as a Backup

Power Source

SuperCaps™ are capacitors with extremely high

capacitance values (e.g., order of 0.47F) for their size.

Figure 15 shows two ways to use a SuperCap as a

backup power source. The SuperCap can be connect-

ed through a diode to the 3V input (Figure 15a); or, if a

5V supply is also available, the SuperCap can be

charged up to the 5V supply (Figure 15b), allowing a

longer backup period. Since VBATT can exceed VCC

while VCC is above the reset threshold, there are no

special precautions when using these µP supervisors

with a SuperCap.

Operation without a

Backup Power Source

These µP supervisors were designed for battery-

backed applications. If a backup battery is not used,

connect BATT, OUT, and VCC together, or use a differ-

ent µP supervisor. See the µP Supervisory Circuits

table at the end of this data sheet.

Replacing the Backup Battery

The backup power source can be removed while VCC

remains valid, without danger of triggering a reset

pulse, provided that BATT is decoupled with a 0.1µF

capacitor to ground. As long as VCC stays above the

reset threshold, battery-backup mode cannot be

entered.

™ LITTLE FOOT is a trademark of Siliconix Inc.

SuperCap is a trademark of Baknor Industries.

______________________________________________________________________________________ 15

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