13

LTC690/LTC691

LTC694/LTC695

Figure 12. Oscillator Configurations

S

APPLICATI

I FOR ATIO

The LTC691 and LTC695 provide an additional output

(Watchdog Output, WDO) which goes low if the watchdog

timer is allowed to time out and remains low until set high

by the next transition on the WDI pin. WDO is also set high

The LTC691 and LTC695 have two additonal pins OSC SEL

and OSC IN, which allow reset active time and watchdog

time-out period to be adjusted per Table 2. Several con-

figurations are shown in Figure 12.

OSC IN can be driven by an external clock signal or an

external capacitor can be connected between OSC IN and

GND when OSC SEL is forced low. In these configurations,

the nominal reset active time and watchdog time-out

period are determined by the number of clocks or set by

the formula in Table 2. When OSC SEL is high or floating,

the internal oscillator is enabled and the reset active time

is fixed at 35ms minimum for the LTC691 and 140ms

minimum for the LTC695. OSC IN selectes between the 1

second and 70ms minimum normal watchdog time-out

periods. In both cases, the time-out period immediately

after a reset is at least 1 second.

Figure 11. Watchdog Time-Out Period and Reset Active Time

AFTER A RESET

WDO

WDI

RESET

690 F11

EXTERNAL CLOCK

INTERNAL OSCILLATOR

1.6 SECOND WATCHDOG

INTERNAL OSCILLATOR

100ms WATCHDOG

EXTERNAL OSCILLATOR

GND

GND

GND

GND

OSC SEL

OSC SEL

OSC SEL

OSC SEL

OSC IN

OSC IN

OSC IN

OSC IN

3

3

3

3

4

4

4

4

8

8

8

8

7

7

7

7

FLOATING

OR HIGH

FLOATING

OR HIGH

LTC691

LTC695

FLOATING

OR HIGH

LTC691

LTC695

LTC691

LTC695

LTC691

LTC695

690 F12

5V

5V

5V

5V