LTC690/LTC691

LTC694/LTC695

7

690fe

PIN FUNCTIONS

PFO: Power Failure Output from C3. PFO remains high

when PFI is above 1.3V and goes low when PFI is below

PFO is forced low.

RESET: Logic Output for μP Reset Control. Whenever

to 5V, reset pulse generator forces RESET to remain active

low for a minimum of 35ms for the LTC690 /LTC691 (140ms

for the LTC694/LTC695). When the watchdog timer is

enabled but not serviced prior to a preset timeout period,

reset pulse generator also forces RESET to active low for a

minimum of 35ms for the LTC690/LTC691 (140ms for the

LTC694/5) for every preset timeout period (see Figure 11).

The reset active time is adjustable on the LTC691/LTC695.

An external pushbutton reset can be used in connection with

the RESET output. See Pushbutton Reset in Applications

Information section.

RESET: RESET is an active high logic ouput. It is the

inverse of RESET.

LOW LINE:LogicOutputfromComparatorC1.LOW⎯LINE

falls below the reset voltage threshold (4.65V typically),

voltage threshold, LOW⎯LINE returns high (see Figure 1).

Table 1).

WDI: Watchdog Input, WDI, is a three level input. Driving

WDI either high or low for longer than the watchdog timeout

period, forces both RESET and WDO low. Floating WDI

disables the watchdog timer. The timer resets itself with

each transition of the watchdog input (see Figure 11).

WDO: Watchdog Logic Output. When the watchdog input

remains either high or low for longer than the watchdog

timeout period, WDO goes low. WDO is set high whenever

there is a transition on the WDI pin, or LOW⎯LINE goes

low. The watchdog timer can be disabled by floating WDI

(see Figure 11).

CE IN: Logic input to the Chip⎯Enable gating circuit. CE IN

can be derived from microprocessor’s address line and/or

decoder output. See Applications Information section and

Figure 5 for additional information.

CE OUT: Logic Output on the Chip⎯Enable Gating Circuit.

voltage threshold CE OUT is forced high (see Figure 5).

OSC SEL: Oscillator Selection Input. When OSC SEL is high

or floating, the internal oscillator sets the reset active time

and watchdog timeout period. Forcing OSC SEL low, allows

OSC IN be driven from an external clock signal or external

capacitor be connected between OSC IN and GND.

OSC IN: Oscillator Input. OSC IN can be driven by

an external clock signal or external capacitor can be

connected between OSC IN and GND when OSC SEL is

forced low. In this configuration the nominal reset active

time and watchdog timeout period are determined by the

number of clocks or set by the formula (see Applications

Information section). When OSC SEL is high or floating,

the internal oscillator is enabled and the reset active time

is fixed at 50ms typical for the LTC691 and 200ms typical

for the LTC695. OSC IN selects between the 1.6 seconds

and 100ms typical watchdog timeout periods. In both

cases, the timeout period immediately after a reset is 1.6

seconds typical.