Datasheet

Data Sheet ADM8690/ADM8691/ADM8695
Rev. C | Page 17 of 24
TYPICAL APPLICATIONS
ADM8690 APPLICATIONS
Figure 30 shows the ADM8690 in a typical power monitoring,
battery backup application. V
OUT
powers the CMOS RAM.
Under normal operating conditions with V
CC
present, V
OUT
is internally connected to V
CC
. If a power failure occurs, V
CC
decays and V
OUT
is switched to V
BATT
, thereby maintaining
power for the CMOS RAM. A
RESET
pulse is also generated
when V
CC
falls below 4.65 V for the ADM8690.
RESET
remains
low for 50 ms after V
CC
returns to 5 V.
+
5
V
0.1µF
WDI
GND
PFI
BATTERY
R1
R2
V
BATT
V
CC
PFO
ADM8690
CMOS RAM
POWER
POWER
MICROPROCESSOR
SYSTEM
NMI
I/O LINE
V
OUT
RESET
RESET
0
0093-030
Figure 30. ADM8690 Typical Application, Circuit A
The watchdog timer input (WDI) monitors an input/output
line from the microprocessor system. This line must be toggled
once every 1.6 sec to verify correct software execution. Failure
to toggle the line indicates that the microprocessor system is
not correctly executing its program and may be tied up in an
endless loop. If this happens, a reset pulse is generated to
initialize the microprocessor.
If the watchdog timer is not needed, the WDI input should be
left floating.
The power-fail input, PFI, monitors the input power supply via
a resistive divider network. The voltage on the PFI input is
compared with a precision 1.3 V internal reference. If the input
voltage drops below 1.3 V, a power-fail output (
PFO
) signal is
generated. This signal warns of an impending power failure and
can be used to interrupt the processor so that the system can be
shut down in an orderly fashion. The resistors in the sensing
network are ratioed to give the desired power-fail threshold
voltage (V
T
).
V
T
= (1.3 R1/R2) + 1.3 V
R1/R2 = (V
T
/1.3) − 1
Figure 31 shows a similar application, but in this case the PFI
input monitors the unregulated input to the 7805 voltage regu-
lator. This circuit provides an earlier warning of an impending
power failure. It is useful with processors that operate at low
speeds or where there are a significant number of housekeeping
tasks to be completed before the power is lost.
+
INPU
T
POWER
V > 8V
0.1µF
WDI
GND
PFI
BATTERY
R1
R2
V
BATT
V
CC
PFO
ADM8690
CMOS RAM
POWER
POWER
MICROPROCESSOR
SYSTEM
NMI
I/O LINE
V
OUT
RESET
RESET
7805
0.1µF
0
0093-031
5V
Figure 31. ADM8690 Typical Application, Circuit B
ADM8691/ADM8695 APPLICATIONS
Figure 32 shows a typical connection for the ADM8691/ADM8695.
CMOS RAM is powered from V
OUT
. When 5 V power is present,
this voltage is routed to V
OUT
. If V
CC
fails, V
BATT
is routed to V
OUT
.
V
OUT
can supply up to 100 mA from V
CC
, but if more current is
required, an external PNP transistor can be added. When V
CC
is
higher than V
BATT
, the BATT ON output goes low, providing up
to 35 mA of base drive for the external transistor. A 0.1 μF capac-
itor is connected to V
OUT
to supply the transient currents for
CMOS RAM. When V
CC
is lower than V
BATT
, an internal 20 Ω
MOSFET connects the backup battery to V
OUT
.
RESET
NC
A0 TO A15
I/O LINE
NMI
RESET
MICROPROCESSOR
SYSTEM
PFI
GND
OSC IN
OSC SEL
WDI
WDO
R1
R2
V
BATT
V
CC
ADM8691/
ADM8695
V
OUT
0.1µF
SYSTEM STATUS
INDICATORS
BATT
ON
3V
BATTERY
INPUT POWER
5V
0.1µF
0.1µF
CMOS
RAM
ADDRESS
DECODE
LOW LINE
RESET
CE
OUT
CE
IN
PFO
00093-032
Figure 32. ADM8691/ADM8695 Typical Application