sp690acp Exar Corporation, sp690acp Datasheet - Page 10

• Manufacturer Part Number: sp690acp
• Description: Low Power Microprocessor Supervisory With Battery Switch-over
• Manufacturer: Exar Corporation

The SP805L/M active-high RESET output is
the inverse of the SP690A/SP692A/SP802 RE-
SET output, and is valid with V
Some µP's, such as Intel's 80C51, require an
active-high reset pulse.
Watchdog Input
The watchdog circuit monitors the µP's activity.
If the µP does not toggle the watchdog input
(WDI) within 1.6sec, a reset pulse is triggered.
The internal 1.6sec timer is cleared by either a
reset pulse or by floating the WDI input. As long
as RESET is asserted or the WDI input is
floating, the timer remains cleared and does not
count. As soon as RESET is released and WDI
is driven high or low, the timer starts counting.
It can detect pulses as short as 50ns.
Figure 13. BACKUP-BATTERY Switchover Block Diagram
Date: 11/29/04
Reset Threshold = 4.65V in SP690A/802L/805L
Reset Threshold = 4.40V in SP692A/802M/805M
V
V
V
V
V
C
C
C
C
C
C
C
C
C
C
>
<
>
<
<
C
R
R
V
R
V
s e
s e
s e
SW1
O
B
B
V
A
A
N
t e
t e
T
t e
T
OUT
T
T
D
T
T
T
I
r h
r h
r h
T
V
s e
s e
s e
I
SP690A/692A Low Power Microprocessor Supervisory with Battery Switch-Over
O
BATT
o h
o h
o h
N
d l
d l
d l
GND
D1
a
a
d n
d n
D3
V
CC
l C
D2
O
O
S
W
s o
p
p
n e
n e
d e
1
CC
down to 1V.
SW2
l C
l C
O
S
W
s o
s o
p
n e
d e
d e
2
10
Power-Fail Comparator
The Power-Fail Comparator can be used as an
under-voltage detector to signal the failing of a
power supply (it is completely separate from the
rest of the circuitry and does not need to be
dedicated to this function). The PFI input is
compared to an internal 1.25V reference. If PFI
is less than 1.25V, PFO goes low. The external
voltage divider drives PFI to sense the
unregulated DC input to the +5V regulator. The
voltage-divider ratio can be chosen such that the
voltage at PFI falls below 1.25V just before the
+5V regulator drops out. PFO then triggers an
interrupt which signals the µP to prepare for
power-down.
When V
comparator is turned off and PFO is forced low
to conserve backup-battery power.
Backup-Battery Switchover
In the event of a brownout or power failure, it
may be necessary to preserve the contents of
RAM. With a backup battery installed at V
the RAM is assured to have power if V
As long as V
V
power switch. Once V
threshold, V
switches to V
through a 5Ω switch only when V
reset threshold and V
When V
connected to V
applied to V
the diode (D1) between V
conduct current from V
more than .6V above V
When V
activated and the internal circuitry will be pow-
ered from the battery Figure 14. When V
just below V
current drawn from V
30µA. When V
V
off and the supply current falls to less than 0.6µA.
OUT
BATT
connects to V
, the internal switchover comparator shuts
BATT
BATT
CC
exceeds the reset threshold, it is
CC
BATT
connects to V
connects to V
BATT
CC
OUT
or V
CC
OUT
exceeds the reset threshold,
Figure 13. During this time,
. V
drops to more than 1V below
, in the backup mode the
, regardless of the voltage
BATT
CC
BATT
BATT
BATT
© Copyright 2004 Sipex Corporation
CC
through a 0.6Ω PMOS
OUT
BATT
, whichever is higher,
falls below the reset
is greater than V
OUT
BATT
.
OUT
to V
connects to V
will be typically
, backup mode is
, the power-fail
OUT
and V
CC
is below the
if V
OUT
CC
BATT
BATT
fails.
CC
will
OUT
CC
is
is
.
,