MAX791 Maxim, MAX791 Datasheet - Page 14

• Manufacturer Part Number: MAX791
• Description: Microprocessor Supervisory Circuit
• Manufacturer: Maxim

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).
Microprocessor Supervisory Circuit
14
Figure 12. SuperCap or MaxCap on VBATT
______________________________________________________________________________________
+5V
1N4148
0.47F
1
VBATT
MAX791
Low-Battery Monitor
GND
V
CC
3
4
V
OUT
2
The MAX791 is not short-circuit protected. Shorting
V
as charging a decoupling capacitor, destroys the
device.
All open-circuit outputs swing between V
rather than V
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:
Figure 13. Alternate CE Gating
__________Applications Information
1) Normal operating mode with all circuitry powered
2) Battery-backup mode where V
3) Battery-backup mode where V
OUT
*MAXIMUM Rp VALUE DEPENDS ON
THE NUMBER OF RAMS.
MINIMUM Rp VALUE IS 1k
up. Typical supply current from V
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.
to ground, other than power-up transients such
CE IN
CC
MAX791
V
and GND.
GND
OUT
CE OUT
Rp*
ACTIVE-HIGH CE
LINES FROM LOGIC
CC
CC
is typically within
CC
CE
CE
CE
CE
OUT
CE
CE
CE
CE
is 60µA, while
is less than
and GND
RAM 1
RAM 2
RAM 3
RAM 4