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LTC2917 View Datasheet(PDF) - Linear Technology

Part Name
Description
MFG CO.
'LTC2917' PDF : 16 Pages View PDF
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LTC2917/LTC2918
APPLICATIONS INFORMATION
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 specified watchdog timeout period,
the capacitor is determined by:
CWT = tWD • 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 VCC
generates a timeout of approximately 1.6s. Connecting
WT to GND disables the watchdog function.
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 VCC pin to allow operation from a high
voltage supply. To operate the part from a supply higher
than 5.7V, the VCC pin must have a series resistor, RCC,
to the supply. See Figure 3. This resistor should be sized
according to the following equation:
VS(MAX) 5.7V
5mA
RCC
VS(MIN) 7V
250μA
where VS(MIN) and VS(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.
(VTRIP = 10.64V)
12V
R2
RCC
1.15M
11k
3.3V
RPU
10k
R1
49.9k
CBYPASS
VCC
0.1μF
LTC2917
VM
RST
SEL1
WDI
SEL2
TOL
GND WT
RT
CWT
CRT
VCC
μP
RST
I/O GND
29178 F03
Figure 3. 12V Supply Monitor Powered From 12V, Utilizing
the Internal Shunt Regulator with 3.3V Logic Out
29178fb
12
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