Low-Battery Monitor

The MAX791 low-battery voltage function monitors

VBATT. Low-battery detection of 2.0V ±0.15V is moni-

tored 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 location 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).

__________Applications Information

The MAX791 is not short-circuit protected. Shorting

as charging a decoupling capacitor, destroys the

device.

If long leads connect to the chip inputs, ensure that

these lines are free from ringing and other conditions

that would forward bias the chip’s protection diodes.

There are three distinct modes of operation:

1) Normal operating mode with all circuitry powered

only leakage currents flow from the battery.

0.7V below VBATT. All circuitry is powered up and

the supply current from the battery is typically less

than 60µA.

VBATT by at least 0.7V. VBATT supply current is

than 1µA max.

Microprocessor Supervisory Circuit

14

______________________________________________________________________________________

MAX791

GND

CE IN

CE

CE

CE OUT

CE

CE

CE

CE

CE

CE

*MAXIMUM Rp VALUE DEPENDS ON

THE NUMBER OF RAMS.

MINIMUM Rp VALUE IS 1k

Ω

ACTIVE-HIGH CE

LINES FROM LOGIC

RAM 1

RAM 2

RAM 3

RAM 4

Rp*

Figure 13. Alternate CE Gating

MAX791

1

0.47F

1N4148

+5V

2

3

GND

VBATT

4

Figure 12. SuperCap or MaxCap on VBATT