SP691A_05 Datasheet, PDF (17/24 Pages) Sipex Corporation – Low Power Microprocessor Supervisory with Battery Switch-Over

CEIN ₁₃
*^{Minimum value of RP is 1kΩ.}
Maximum value of RP is dependent
on the connected number of RAMs, n.
2 ^VOUT
GND
4
CEOUT
12
RP*
CE
RAM1
CE
Active-HIGH
CE Logic Lines
for Memory Devices
CE
RAM2
CE
CE
RAM3
CE
CE
RAMn
CE
Figure 26. Alternate Chip Enable Gating
+5V
VCC
PFI
VIN
R1
R3
*C1
R2
PFO
*optional
GND
connect to µP
VTRIP =
1.25
R2
R1 + R2
1.25
R2
=
VL
- 1.25
R1
+
5.0 - 1.25
R3
VH =
1.25
R2 || R3
PFO
R1 + R2 || R3
+5V
0V
VIN
0V
VL VTRIP VH
Figure 27. Adding Hysteresis to the Power-Fail
Comparator
and connect CE_OUT to the CE input of each
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_IN is near the power-fail comparator
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_IN
falls to the desired trip point (V_TRIP). Resistor
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
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 VCC
voltage, and resistors R1 and R2.
Backup-Battery Replacement
The backup battery may be disconnected while
VCC is above the reset threshold. No precautions
are necessary to avoid spurious reset pulses.
Date: 4/18/05
SP691A/693A/800L/800M Low Power Microprocessor Supervisor with Battery Switch-Over © Copyright 2005 Sipex Corporation
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