SP791 데이터 시트보기 (PDF) - Signal Processing Technologies

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SP791

Low Power Microprocessor Supervisory with Battery Switch-Over

Signal Processing Technologies 

RESET

THRESHOLD

Vcc

RESET

CE IN

200ms TYP

CE OUT

SECOND CE PULSE ABSENT WHEN VBATT < 2V

Figure 11. Backup-Battery Monitor Timing Diagram

LOW-BATTERY MONITOR

The SP791 low-battery voltage function moni-

tors VBATT. Low-battery detection of 2.0V

±0.15V is monitored only during the reset-

timeout period (200ms) that occurs either after

a normal power-up sequence or after the MR

reset input has been returned to its high state. If

the battery voltage is below 2.0V, the second

CE pulse is inhibited after reset timeout. If the

battery voltage is above 2.0V, all CE pulses are

allowed through the CE gate after the reset

timeout period. To use this function, after the

200ms reset delay, write 00 (HEX) to a loca-

tion using the first CE pulse, and write FF (HEX)

to the same location using the second CE pulse

following RESET going inactive on power-up.

The contents of the memory then indicates a

good battery (FF) or a low battery (00),

Figure 11.

TYPICAL APPLICATIONS

The SP791 is not short-circuit protected. Short-

ing VOUT to ground, other than power-up tran-

sients such as charging a decoupling capacitor,

may destroy the device. All open-circuit out-

puts swing between VOUT and GND rather than

VCC and GND. If long leads connect to the chip

inputs, ensure that these lines are free from ring-

ing and other conditions that would forward bias

the chip's protection diodes.

+5V

1N4148

0.47F

3

Vcc

1

VBATT

2

VOUT

Corporation

GND

4

Figure 12. High Capacity Capacitor on VBATT

There are three distinct modes of operation:

1) Normal operating mode with all circuitry

powered up from VCC. Typical supply

current from VCC is 40µA, 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 from VBATT, and the supply

current is typically less than 40µA.

3) Battery-backup mode where VCC is less than

VBATT by at least 0.7V. VBATT supply

current is less than 1µA.

USING HIGH CAPACITY CAPACITOR

WITH THE SP791

VBATT has the same operating voltage range as

VCC, and the battery-switchover threshold volt-

ages are typically +30mV centered at VBATT,

allowing use of a capacitor and a simple charg-

ing circuit as a backup source (see Figure 12).

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.

SP791DS/08

™ - REGISTERED TRADEMARK OF BAKNOR INDUSTRIES

SP791 Low Power Microprocessor Supervisory with Battery Switch-Over

14

© Copyright 2000 Sipex Corporation

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