SP691ACN SIPEX [Sipex Corporation], SP691ACN Datasheet - Page 17

• Manufacturer Part Number: SP691ACN
• Description: Low Power Microprocessor Supervisory with Battery Switch-Over
• Manufacturer: SIPEX [Sipex Corporation]

Figure 26. Alternate Chip Enable Gating
and connect CE
memory device as shown in Figure 26. The CE
input of each part then connects directly to the
chip-select logic, which does not have to gated
by the SP691A/693A/800L/800M devices.
Adding Hysteresis to the Power-Fail
Comparator
Hysteresis adds noise margin to the power-fail
comparator and prevents repeated triggering of
PFO when V
trip point. Figure 27 shows how to add hysteresis
to the power-fail comparator. Select the ratio of
R1 and R2 such that PFI sees 1.25V when V
falls to the desired trip point (V
R3 adds hysteresis. It will typically be an order
of magnitude greater than R1 or R2. The
current through R1 and R2 should be at least
1µA to ensure that the 25nA (max) PFI input
current does not shift the trip point. R3 should
Date: 5/25/04
for Memory Devices
GND
CE
CE Logic Lines
Active-HIGH
IN
13
4
IN
12
2
SP691A/693A/800L/800M Low Power Microprocessor Supervisor with Battery Switch-Over © Copyright 2004 Sipex Corporation
is near the power-fail comparator
CE
V
OUT
OUT
OUT
*
to the CE input of each
Minimum value of R
Maximum value of R
on the connected number of RAMs, n.
R
P
*
CE
CE
CE
CE
CE
CE
CE
CE
TRIP
P
P
is 1kΩ.
is dependent
). Resistor
RAM
RAM
RAM
RAM
3
2
n
1
IN
17
Figure 27. Adding Hysteresis to the Power-Fail
Comparator
be larger than 10kΩ to prevent it from loading
down the PFO pin. Capacitor C1 adds additional
noise rejection.
Monitoring a Negative Voltage
The power-fail comparator can be used to monitor
a negative supply voltage using the circuit shown
in Figure 28. When the negative supply is valid,
PFO is low. When the negative supply voltage
drops, PFO goes high. This circuit's accuracy is
affected by the PFI threshold tolerance, the V
voltage, and resistors R1 and R2.
Backup-Battery Replacement
The backup battery may be disconnected while
V
are necessary to avoid spurious reset pulses.
+5V
CC
0V
+5V
PFO
is above the reset threshold. No precautions
0V
V
GND
CC
1.25
PFO
PFI
R
V
2
L
V
V
=
H
V
TRIP
V
=
TRIP
L
=
- 1.25
R
R
V
R
1
3
H
1
R
R
+ R
1.25
1.25
1
2
R
connect to µP
+ R
|| R
+
2
2
*C
|| R
5.0 - 1.25
2
3
1
R
3
*optional
3
V
IN
R
R
V
1
2
IN
CC