MAX691A MAXIM [Maxim Integrated Products], MAX691A Datasheet - Page 13

• Manufacturer Part Number: MAX691A
• Description: Microprocessor Supervisory Circuits
• Manufacturer: MAXIM [Maxim Integrated Products]

Figure 10. Alternate CE Gating
If using separate power supplies for V
VBATT must be less than 0.3V above V
above the reset threshold. As described in the previ-
ous section, if VBATT exceeds this limit and power is
lost at V
V
switch until the circuit is broken (Figure 8).
Using memory devices with both CE and CE inputs
allows the CE loop to be bypassed. To do this, con-
nect CE IN to ground, pull up CE OUT to V
connect
device (Figure 10). The CE input of each part then
connects directly to the chip-select logic, which does
not have to be gated.
Hysteresis adds a noise margin to the power-fail com-
parator and prevents repeated triggering of PFO when
V
11 shows how to add hysteresis to the power-fail com-
CC
IN
*MAXIMUM Rp VALUE DEPENDS ON
is near the power-fail comparator trip point. Figure
THE NUMBER OF RAMS.
MINIMUM Rp VALUE IS 1k
via the VBATT-to-V
CC
–
CE OUT to the CE input of each memory
, current flows continuously from VBATT to
CE IN
Using Separate Power Supplies
MAX800M
MAX691A
MAX693A
MAX800L
V
GND
OUT
Alternate Chip-Enable Gating
______________________________________________________________________________________
CE OUT
Rp*
Adding Hysteresis to the
OUT
Power-Fail Comparator
Microprocessor Supervisory Circuits
diode and the V
for VBATT and V
ACTIVE-HIGH
FROM LOGIC
CE LINES
CC
CC
CE
CE
CE
CE
CE
CE
CE
CE
when V
and VBATT,
OUT
OUT
-to-V
RAM 1
RAM 2
RAM 3
RAM 4
, and
CC
CC
CC
is
Figure 11. Adding Hysteresis to the Power-Fail Comparator
Figure 12. Monitoring a Negative Voltage
PFO
NOTE: V
5 - 1.25 = 1.25 - V
5V
0V
R1
+5V
PFO
R1
R2
5V
0V
TRIP
V
V
TRIP
H
V
0V
= 1.25/
IN
IS NEGATIVE
= 1.25 R1 + R2
R2
R2
R1
TRIP
R1 + R2 I I R3
V-
TO P
R2 I I R3
R2
C1*
R3
V
V-
TRIP
PFI
PFI
PFO
V
MAX800M
MAX691A
MAX693A
MAX800L
L
MAX800M
V
MAX691A
MAX693A
MAX800L
L
- 1.25 + 5 - 1.25 = 1.25
GND
R1
V
GND
+5V
V
CC
V
CC
TRIP
V
IN
V
PFO
R3
H
0V
*OPTIONAL
R2
13