ltc1726-5 Linear Technology Corporation, ltc1726-5 Datasheet - Page 8

• Manufacturer Part Number: ltc1726-5
• Description: Triple Supply Monitor And ?p Supervisor With Adjustable Reset And Watchdog Timer
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC1726
Figure 3 contains a simple circuit for 5V systems that can’t
risk the RST output going high in the 3.1V to 4.15V range
(possibly due to very slow rise time on the 5V supply).
Diode D1 powers the LTC1726-5 while dropping ≈ 0.6V
from the V
internal override circuit from being activated. Without the
override circuit active, the RST pin stays low until V
reaches V
8
5V
0.1µF
Figure 3. LTC1726-5 Monitoring a Single 5V
Supply. D1 Used to Avoid RST High Near 3.1V
to 4V (See Figure 2).
RT5
CC5
Figure 4. RST Output Voltage
Characteristics of the Circuit in Figure 3
5
4
3
2
1
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
≅ 4.675V. (See Figure 4.)
pin to the V
V
10k PULL-UP FROM RST TO V
T
D1
A
CC5
= 25°C
= V
1
2
3
4
U
CCA
D1: MMBD914 OR EQUIVALENT
PINS 5, 7 AND 8 NOT SHOWN FOR CLARITY
V
V
V
GND
CC3
CC5
CCA
= 0V TO 5V
LTC1726-5
CC3
V
U
CC5
RST
(V)
pin. This prevents the part’s
6
CC5
W
R1
10k
1726 F04
TO SYSTEM
RESET
U
1726 F03
CC5
LTC1726-2.5 Override Functions
The V
This is an obvious solution since the trip points for V
and V
V
V
operating off of a 2.5V supply will result in the continuous
assertion of RST.
Watchdog Timer
The watchdog circuit monitors a µP’s activity. The µP is
required to change the logic state of the WDI pin on a
periodic basis in order to clear the watchdog timer and
prevent the LTC1726 from issuing a reset.
During power-up, the watchdog timer remains cleared
while reset is asserted. As soon as the reset timer times out,
the watchdog timer is started. The watchdog timer will
continue to run until a transition is detected on the WDI
input or until the watchdog timer times out. Once the
watchdog timer times out, the internal circuitry asserts the
reset and starts the reset timer. When the reset timer times
out and reset is deasserted, the watchdog timer is again
started. If no WDI transition is received within the watchdog
time-out period, the reset will be reasserted at the end of the
watchdog time-out period. If a transition is received on the
WDI input during the watchdog time-out period, the watch-
dog timer will be restarted and reset will remain deasserted.
CCA
CC3
. Likewise, the V
CCA
must always be used. Tying V
CC25
pin, if unused, can be tied to either V
will always be greater than the trip point for
CC25
, if unused, can be tied to V
CC3
to V
CC3
CC25
or V
CC25
1726fb
CC3
and
CC3
.
.