sp791 Exar Corporation, sp791 Datasheet - Page 15

• Manufacturer Part Number: sp791
• Description: Low Power Microprocessor Supervisory With Battery Switch-over
• Manufacturer: Exar Corporation

SP791DS/08
Leakage current through the capacitor charging
diode and the SP791 internal power diode even-
tually discharges the capacitor to V
V
threshold and power is lost at V
on V
reaches the reset threshold; the SP791 then
switches to battery-backup mode.
USING SEPARATE POWER SUPPLIES
FOR V
If using separate power supplies for V
V
when V
described in the previous section, if V
ceeds this limit and power is lost at V
flows continuously from V
V
switch until the circuit is broken.
ALTERNATIVE CHIP-ENABLE GATING
Using memory devices with CE and CE inputs
allows the SP791 CE loop to be bypassed. To do
this, connect CE IN to ground, pull up CE
to V
of each memory device as shown in Figure 13.
The CE input of each part then connects directly
to the chip-select logic, which does not have to
be gated by the SP791.
Figure 13. Alternate CE Gating
CC
BATT
BATT
*
MAXIMUM Rp VALUE DEPENDS ON
THE NUMBER OF RAM DEVICES.
MINIMUM Rp VALUE IS 1K
OUT
BATT
and V
, V
-to-V
BATT
, and connect CE OUT to the CE input
CC
BATT
discharges through V
BATT
is above the reset threshold. As
OUT
AND V
CE
must be less than 0.3V above V
IN
start from 0.5V above the reset
diode and the V
V
GND
OUT
Corporation
SP791 Low Power Microprocessor Supervisory with Battery Switch-Over
CE
CC
Rp
OUT
*
ACTIVE-HIGH CE
LINES FROM LOGIC
BATT
CC
CC
to V
, the capacitor
OUT
until V
CC
CC
CC
CE
CE
CE
CE
CE
CE
CE
CE
BATT
. Also, if
, current
-to-V
CC
via the
RAM 1
RAM 2
RAM 3
RAM 4
BATT
OUT
and
ex-
CC
CC
15
ADDING HYSTERESIS TO THE
POWER-FAIL COMPARATOR
Hysteresis adds a noise margin to the power-fail
comparator and prevents repeated triggering of
PFO when VIN is near the trip point. Figure 14
shows how to add hysteresis to the power-fail
comparator. Select the ratio of R1 to R2 such
that PFI sees 1.25V when VIN falls to the de-
sired trip point (VTRIP). Resistor R3 adds hys-
teresis. It will typically be an order of magni-
tude greater than R1 or R2. The current through
R1 and R2 should be at least 1 A to ensure that
the 25nA (max) PFI input current does not shift
the trip point. R3 should be larger than 10k to
prevent it from loading down the PFO pin. Ca-
pacitor C1 adds additional noise rejection.
MONITORING A NEGATIVE VOLTAGE
The power-fail comparator can be used to moni-
tor a negative supply voltage using the circuit
shown in Figure 15. When the negative supply
is valid, PFO is low. When the negative supply
voltage drops, PFO goes high. This circuit's ac-
curacy is affected by the PFI threshold tolerance,
the V
BACKUP-BATTERY REPLACEMENT
The backup battery may be disconnected while
V
are necessary to avoid spurious reset pulses.
Figure 14. Adding Hysteresis to the Power-Fail Comparator
CC
PFO
is above the reset threshold. No precautions
+5V
R1
R2
OV
CC
VIN
OV
V
V
voltage, and resistors R1 and R2.
H
TRIP
= 1.25
R3
TO P
= 1.25 *
C1*
(
R1 + R2 II R3
(
R1 + R2
R2 II R3
R2
PFI
PFO
)
V
)
+5V
L
Vcc
GND
V
Corporation
TRIP
© Copyright 2000 Sipex Corporation
V
V
L
H
- 1.25
R1
* OPTIONAL FOR ADDITIONAL
+
NOISE REJECTION
V
5 - 1.25
IN
R3
=
1.25
R2