Datasheet

LT3689/LT3689-5
24
3689fe
For more information www.linear.com/LT3689
In both watchdog modes, when WDO is asserted, the reset
timer is enabled. Any WDI pulses that appear while the
reset timer is running are ignored. When the reset timer
expires, the WDO is allowed to go high again. Therefore,
if no input is applied to the WDI pin, then the watchdog
circuitry produces a train of pulses on the WDO pin. The
high time of this pulse train is equal to the timeout period,
and low time is equal to the reset period. Also, WDO and
RST cannot be logic low simultaneously. If WDO is low
and RST goes low, then WDO will go high.
The WDE pin allows the user to turn on and off the watch-
dog function. Do not leave this pin open. Tie it high or
low to turn watchdog off or on, respectively. The W/T pin
enables/disables the window/timeout mode. Leaving this
pin open is fine and will put the watchdog in window mode.
It has a weak pull-down to ground. The WDI pin has an
internal 2µA weak pull-up that keeps the WDI pin high. If
watchdog is disabled, leaving this pin open is acceptable.
Selecting the Reset Timing Capacitors
The reset timeout period is adjustable in order to accom-
modate a variety of microprocessor applications. The
reset timeout period, (t
RST
), is adjusted by connecting a
capacitor, C
POR
, between the C
POR
pin and ground. The
value of this capacitor is determined by:
C
POR
= t
RST
432
pF
m s
This equation is accurate for reset timeout periods of
5ms, or greater. To program faster timeout periods, see
the Reset Timeout Period vs Capacitance graph in the
Typical Performance Characteristics section. Leaving the
C
POR
pin unconnected will generate a minimum reset
timeout of approximately 25µs. Maximum reset timeout
is limited by the largest available low leakage capacitor.
The accuracy of the timeout period will be affected by
APPLICATIONS INFORMATION
Figure 10. Reset Timer Waveforms
Figure 11. Window Watchdog Waveforms (W/T = Low)
Figure 12. Timeout Watchdog Waveforms (W/T = High)
50ms/DIV
V
OUT
2V/DIV
C
POR
1V/DIV
RST
2V/DIV
3689 F10
t
RST
= 165ms
C
POR
= 71nF
5ms/DIV
C
WDT
1V/DIV
C
POR
1V/DIV
WDI
5V/DIV
WDO
2V/DIV
3689 F11
C
WDT
= 10nF, t
WDL
= 5.8ms
100ms/DIV
C
WDT
1V/DIV
C
POR
1V/DIV
WDI
5V/DIV
WDO
2V/DIV
3689 F12
t
RST
= 165ms, C
POR
= 71nF
t
WDU
= 180ms, C
WDT
= 10nF