LTC1727 datasheet(12/16 Pages) LINER

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LTC2917/LTC2918

29178f

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 ⎯R⎯S⎯T

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.

APPLICATIONS INFORMATION

Manual Reset (LTC2918 Only)

The LTC2918 includes the ⎯M⎯R pin for applications where a

manual reset is desired. ⎯M⎯R is internally pulled up, making

it ready to interface with a push button with no external

components required. Asserting ⎯M⎯R low when ⎯R⎯S⎯T is high

initiates a reset, resulting in ⎯R⎯S⎯T 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:

SMAX

SMIN

()

(

)

−

≤≤

−

250

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.

Figure 3. 12V Supply Monitor Powered From 12V, Utilizing

the Internal Shunt Regulator with 3.3V Logic Out

RCC

11k

12V

3.3V

CRT

CBYPASS

0.1

μF

RPU

10k

29178 F03

SEL1

GND

SEL2

TOL

VCC

(VTRIP = 10.64V)

VCC

LTC2917

μP

RST

WDI

I/O

GND

1.15M

49.9k

CWT

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