LTC690 Linear Technology, LTC690 Datasheet - Page 12

• Manufacturer Part Number: LTC690
• Description: Microprocessor Supervisory Circuits
• Manufacturer: Linear Technology

LTC690/LTC691
LTC694/LTC695
A
of LT1086-5 (4.75V + 1.5V) and V
Choose R3 = 300k and R1 = 51k. Also select R4 = 10k
which is much smaller than R3.
R2 = 9.7k , Choose nearest 5% resistor 10k and recalcu-
late V
The 10.7ms allows enough time to execute shutdown
procedure for microprocessor and 831mV of hysteresis
would prevent PFO from going low due to the noise of V
Example 2: The circuit in Figure 9 can be used to measure
the regulated 5V supply to provide early warning of power
failure. Because of variations in the PFI threshold, this
circuit requires adjustment to ensure the PFI comparator
trips before the reset threshold is reached. Adjust R5 such
that the PFO output goes low when the V
the desired level (e.g., 4.85V).
Monitoring the Status of the Battery
C3 can also monitor the status of the memory back-up
battery (Figure 10). If desired, the CE OUT can be used to
apply a test load to the battery. Since CE OUT is forced high
in battery back-up mode, the test load will not be applied
to the battery while it is in use, even if the microprocessor
is not powered.
12
PPLICATI
R3 5.88 R1
V
(7.32V – 6.25V)
V
V
7.5V = 1.3V 1+
V
H
L
HYSTERESIS
HYSTERESIS
100mV/ms
L
,
1.3V 1
1.3V 1
= 8.151V – 7.32V = 831mV
O
51
10k
10k
51k
5V
51
U
R2
k
R3
k
10.7ms
S
R1
–
–
(5V – 1.3V)51
300k
51k
I FOR ATIO
(5V – 1.3V)51
1.3V(310k
U
1 3 310
850V
. (
V
HYSTERESIS
8.151V
W
k
)
)
k
CC
k
supply reaches
7 32
. V
= 850mV.
U
IN
.
Watchdog Timer
The LTC690 family provides a watchdog timer function to
monitor the activity of the microprocessor. If the micro-
processor does not toggle the Watchdog Input (WDI)
within a seleced time-out period, RESET is forced to active
low for a minimum of 35ms for the LTC690/LTC691
(140ms for the LTC694/LTC695). The reset active time is
adjustable on the LTC691/LTC695. Since many systems
can not service the watchdog timer immediately after a
reset, the LTC691 and LTC695 have longer time-out
period (1.0 second minimum) right after a reset is issued.
The normal time-out period (70ms minimum) becomes
effective following the first transition of WDI after RESET
is inactive. The watchdog time-out period is fixed at 1.0
second minimum on the LTC690 and LTC694. Figure 11
shows the timing diagram of watchdog time-out period
and reset active time. The watchdog time-out period is
restarted as soon as RESET is inactive. When either a high-
to-low or low-to-high transition occurs at the WDI pin
prior to time-out, the watchdog time is reset and begins to
time out again. To ensure the watchdog time does not time
out, either a high-to-low or low-to-high transition on the
WDI pin must occur at or less than the minimum time-out
period. If the input to the WDI pin remains either high or
low, reset pulses will be issued every 1.6 seconds typi-
cally. The watchdog time can be deactivated by floating the
WDI pin. The timer is also disabled when V
the reset voltage threshold or V
Figure 10. Back-Up Battery Monitor with Optional Test Load
3V
OPTIONAL TEST LOAD
R1
1M
R2
1M
20K
R
L
V
PFI
CE OUT
BATT
LTC691
LTC695
V
5V
CC
GND
CE IN
BATT
PFO
690 F10
.
LOW-BATTERY SIGNAL
TO P I/O PIN
I/O PIN
CC
falls below