sp791 Exar Corporation, sp791 Datasheet - Page 13

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

INPUT/OUTPUT STATES IN
BATTERY-BACKUP MODE
SP791DS/08
Table 1. Input/Output states in Battery-Backup mode
BATTERY-BACKUP MODE
The SP791 requires two conditions to switch to
battery-backup mode: 1) V
the reset threshold; 2) V
V
outputs in battery-backup mode.
BATTERY ON OUTPUT
The Battery On Output (BATT ON) indicates
the status of the internal V
comparator, which controls the internal V
V
(ignoring the small hysteresis effect), BATT ON
is a logic low. For V
ON is a logic high. Use BATT ON to indicate
battery-switchover status or to supply base drive
to an external pass transistor for higher-current
applications (see Typical Operating Circuit).
To enter the Battery-Backup mode, V
the Reset threshold and less than V
PIN
10
11
12
13
14
15
16
BATT
BATT
1
2
3
4
5
6
7
8
9
. Table 1 lists the status of the inputs and
switches. For V
BATT ON
NAME
LOWLINE
RESET
CE
WDPO
V
WDO
CE
V
GND
SWT
PFO
WDI
V
PFI
MR
BATT
OUT
CC
OUT
IN
Supply current is 1 A maximum
When V
V
through an Internal PMOS switch.
Battery-switchover comparator
monitors V
V
GND-0V reference for all signals.
Logic high. The open-circuit output is
equal to V
The power-fail comparator is disabled
PFO is forced low.
The power-fail comparator is disabled
SWT is Ignored.
MR is ignored.
WDI is ignored, and goes high
Logic high. The open-circuit output
voltage is equal to V
High Impedance.
Logic high. The open-circuit output
voltage is equal to V
Logic low.
Logic high. The open-circuit output
voltage is equal to V
Logic low.
impedance.
OUT
CC
CC
SP791 Low Power Microprocessor Supervisory with Battery Switch-Over
is disconnected from V
is connected to V
less than V
CC
CC
CC
OUT
CC
STATUS
< V
CC
greater than V
/battery-switchover
CC
BATT
.
for active switchover.
BATT
CC
must be below
.
must be below
must be less than
-1.2V
OUT
OUT
OUT
BATT
BATT
.
.
.
, BATT
OUT
CC
BATT
and
13
INPUT SUPPLY VOLTAGE
The Input Supply Voltage (V
regulated +5V source. V
via a parallel diode and a large PMOS switch
(Figure 10). The switch carries the entire
current load for currents less than 250mA.
The parallel diode carries any current in excess
of 250mA. The maximum continuous current
is 250mA, but power-on transients may reach a
maximum of 1A.
BACKUP-BATTERY INPUT
The Backup-Battery Input (V
to V
diode are much smaller. Continuous current
should be limited to 25mA and peak currents
(only during power-up) limited to 250mA. The
reverse leakage of this input is less than 1 A
over temperature and supply voltage.
OUTPUT SUPPLY VOLTAGE
The Output Supply Voltage (V
the current to the external system and internal
circuitry. All open-circuit outputs will, for ex-
ample, assume the V
states rather than the V
mum source current of 250mA, V
cally be 200mV below V
decoupled with 0.1 F capacitor.
Figure 10. V
V
CC
BATT
Vcc
, except the PMOS switch and parallel
CC
and V
BATT
Corporation
OUT
-to-V
© Copyright 2000 Sipex Corporation
CC
CC
voltage. At the maxi-
CC
OUT
voltage in their high
. V
connects to V
Switch
2
OUT
CC
BATT
OUT
) should be a
OUT
) supplies all
0.1 F
) is similar
should be
will typi-
V
OUT
OUT