Mic ro p ro c e s s o r S u p e rvis o ry Circ u it
Rp*
CE
CE
RAM 1
RAM 2
RAM 3
RAM 4
+5V
V
OUT
3
CE IN
CE OUT
V
CC
1N4148
CE
CE
MAX791
1
2
VBATT
V
OUT
CE
CE
0.47F
MAX791
GND
MAX791
CE
CE
GND
4
*MAXIMUM Rp VALUE DEPENDS ON
THE NUMBER OF RAMS.
MINIMUM Rp VALUE IS 1kΩ
ACTIVE-HIGH CE
LINES FROM LOGIC
Figure 12. SuperCap or MaxCap on VBATT
Figure 13. Alternate CE Gating
Lo w -Ba t t e ry Mo n it o r
__________Ap p lic a t io n s In fo rm a t io n
The MAX791 low-b a tte ry volta g e func tion 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
volta g e is a b ove 2.0V, a ll CE p uls e s a re a llowe d
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).
The MAX791 is not short-circuit protected. Shorting
V
OUT
to ground, other than power-up transients such
a s c ha rg ing a d e c oup ling c a p a c itor, d e s troys the
device.
All open-circuit outputs swing between V
and GND
OUT
rather than V and GND.
CC
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
up. Typical supply current from V is 60µA, while
CC
only leakage currents flow from the battery.
2) Battery-backup mode where V is typically within
CC
0.7V below VBATT. All circuitry is powered up and
the supply current from the battery is typically less
than 60µA.
3) Ba tte ry-b a c kup mod e whe re V
is le s s tha n
CC
VBATT by at least 0.7V. VBATT supply current is
than 1µA max.
14 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
