LTC2901 Linear Technology, LTC2901 Datasheet - Page 12

• Manufacturer Part Number: LTC2901
• Description: Programmable Quad Supply Monitor with Adjustable Reset and Watchdog Timers
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Pull-Up Current vs V2 curve). Output rise time (10% to
90%) is estimated using:
where R
The on-resistance as a function of the V2 voltage at room
temperature is estimated using:
with V2 = 3.3V, R
capacitance, the rise time is 86 s. If the output needs to
pull up faster and/or to a higher voltage, a smaller
external pull-up resistor may be used. Using a 10k pull-
up resistor, the rise time is reduced to 3.3 s for a 150pF
load capacitance.
The LTC2901-2 has an active pull-up to V2 on the RST
output. The typical performance curve (RST Pull-Up Cur-
rent vs V2 curve) demonstrates that the pull-up current is
somewhat linear versus the V2 voltage and R
mated to be approximately 625 . A 150pF load capaci-
tance makes the rise time about 206ns.
Watchdog Timer
The watchdog circuit typically monitors a P’s activity. The
a periodic basis in order to clear the watchdog timer and
prevent the WDO pin from going low. Whenever RST is low,
the watchdog timer is cleared and WDO is set high. The
watchdog timer is started when RST pulls high. Subsequent
edges received on the WDI pin will clear the watchdog timer.
The timer will continue to run until the watchdog timer times
out. Once the watchdog timer times out, internal circuitry
will bring the WDO pin low. WDO will remain low and the
watchdog timer will remain cleared until the next edge is
received on the WDI pin or until RST goes low.
To disable the watchdog timer, simply ground the CWT pin
(Pin 9). With CWT held at ground, any reset event will force
WDO high indefinitely. It is safe to leave the WDI pin (Pin 8)
unconnected because the weak internal pull-up (10 A typ)
will pull WDI high. Tying WDI to V1 or ground is also
allowed, but grounding the WDI pin will force the pull-up
current to be drawn continuously.
LTC2901
12
P is required to change the logic state of the WDI pin on
t
R
RISE
PU
PU
2.2 • R
6 10
V
is the on-resistance of the pull-up transistor.
•
2 1
–
5
PU
PU
U
• C
is about 260k. Using 150pF for load
LOAD
U
W
U
PU
is esti-
Selecting the Reset Timing Capacitor
The reset time-out period is adjustable in order to accom-
modate a variety of microprocessor applications. The
reset time-out period, t
capacitor, C
value of this capacitor is determined by:
with C
millisecond of delay can also be expressed as C
217.4 (pF/ms).
Leaving the CRT pin unconnected will generate a mini-
mum reset time-out of approximately 50 s. Maximum
reset time-out is limited by the largest available low
leakage capacitor. The accuracy of the time-out period will
be affected by capacitor leakage (the nominal charging
current is 2 A) and capacitor tolerance. A low leakage
ceramic capacitor is recommended.
Selecting the Watchdog Timing Capacitor
The watchdog time-out period is adjustable and can be
optimized for software execution. The watchdog time-out
period, t
between the CWT pin and ground. Given a specified
watchdog time-out period, the capacitor is determined by:
with C
millisecond of delay can also be expressed as C
50 (pF/ms).
Leaving the CWT pin unconnected will generate a mini-
mum watchdog time-out of approximately 200 s. Maxi-
mum time-out is limited by the largest available low
leakage capacitor. The accuracy of the time-out period will
be affected by capacitor leakage (the nominal charging
current is 2 A) and capacitor tolerance. A low leakage
ceramic capacitor is recommended.
Monitoring Power Supply Controller Activity
Figure 7 demonstrates how the LTC2901 can be used to
monitor switcher activity. The monitor is configured to
supervise 3.3V, 2.5V, 1.8V and one adjustable input.
C
C
RT
WT
RT
WT
= t
= t
WD
in Farads and t
in Farads and t
RST
WD
, is adjusted by connecting a capacitor, C
RT
• 217.4 • 10
• 50 • 10
, between the CRT pin and ground. The
–9
RST
RST
WD
–9
, is adjusted by connecting a
in seconds. The C
in seconds. The C
WT
RT
value per
value per
2901is sn2901
WT/ms
RT/ms
WT
=
=
,