Datasheet

LTC2917/LTC2918
12
29178fb
LTC2917-B/LTC2918-B
For applications in which reliability is even more critical,
the LTC2917-B/LTC2918-B implements a windowed watch-
dog function by adding a lower boundary condition to the
standard watchdog function. If the WDI input receives a
falling edge prior to the watchdog lower boundary, the
part considers this a watchdog failure, and asserts RST
low (releasing again after the reset timeout period as
described above). The LTC2917-B/LTC2918-B WDI input
only responds to falling edges.
Setting the Watchdog Timeout Period
The watchdog timeout period is adjustable and can be
optimized for software execution. The watchdog timeout
period is adjusted by connecting a capacitor between WT
and ground. Given a specifi ed watchdog timeout period,
the capacitor is determined by:
C
WT
= t
WD
• 13.8 [nF/s]
For example, using a standard capacitor value of 0.047μF
would give a 3.4s watchdog timeout period.
Leaving WT open with no external capacitor generates
a timeout of approximately 3.2ms. Shorting WT to V
CC
generates a timeout of approximately 1.6s. Connecting
WT to GND disables the watchdog function.
APPLICATIONS INFORMATION
Manual Reset (LTC2918 Only)
The LTC2918 includes the MR pin for applications where a
manual reset is desired. MR is internally pulled up, making
it ready to interface with a push button with no external
components required. Asserting MR low when RST is high
initiates a reset, resulting in RST being asserted low for
the reset timeout time.
Shunt Regulator
The LTC2917 and LTC2918 contain an internal 6.2V shunt
regulator on the V
CC
pin to allow operation from a high
voltage supply. To operate the part from a supply higher
than 5.7V, the V
CC
pin must have a series resistor, R
CC
,
to the supply. See Figure 3. This resistor should be sized
according to the following equation:
VV
mA
R
VV
A
SMAX
CC
SMIN() ()
.−
≤≤
−57
5
7
250μ
where V
S(MIN)
and V
S(MAX)
are the operating minimum
and maximum of the supply.
As an example, consider operation from an automobile
battery which might dip as low as 10V or spike to 60V. We
must then pick a resistance between 10.86k and 12k.
Figure 3. 12V Supply Monitor Powered From 12V, Utilizing
the Internal Shunt Regulator with 3.3V Logic Out
R
CC
11k
12V 3.3V
C
RT
C
BYPASS
0.1μF
R
PU
10k
29178 F03
VM
SEL1 GND
SEL2
RT
TOL
V
CC
(V
TRIP
= 10.64V)
V
CC
LTC2917 μP
RST
RST
WDI I/O
GND
R2
1.15M
R1
49.9k
C
WT
WT