Datasheet

12
LTC2901
2901fb
APPLICATIO S I FOR ATIO
WUUU
In a negative adjustable application, the minimum value
for R4 is limited by the sourcing capability of V
REF
(±1mA).
With no other load on V
REF
, R4 (minimum) is:
1.21V ÷ 1mA = 1.21kΩ.
Tables 2 and 3 offer suggested 1% resistor values for
various adjustable applications.
Once the resistor divider is set in the 5% tolerance mode
(LTC2901-3/LTC2901-4), there is no need to change the
divider for the 10% mode because the internal and exter-
nal reference is scaled accordingly, moving the trip point
by –5%.
Although all four supply monitor comparators have built-
in glitch immunity, bypass capacitors on V1 and V2 are
recommended because the greater of V1 or V2 is also the
V
CC
for the device. Filter capacitors on the V3 and V4
inputs are allowed.
Power-Down
On power-down, once any of the V
X
inputs drop below
their threshold, RST and COMPX are held at a logic low.
A logic low of 0.4V is guaranteed until both V1 and V2
drop below 1V. If the bandgap reference becomes invalid
(V
CC
< 2V typ), the part will reprogram once V
CC
rises
above 2.4V max.
Monitor Output Rise and Fall Time Estimation
All of the outputs (RST, COMPX, WDO) have strong pull-
down capability. If the external load capacitance (C
LOAD
)
for a particular output is known, output fall time (10% to
90%) is estimated using:
t
FALL
≈ 2.2 • R
PD
• C
LOAD
where R
PD
is the on-resistance of the internal pull-down
transistor. The typical performance curve (V
OL
vs I
SINK
)
demonstrates that the pull-down current is somewhat
linear versus output voltage. Using the 25°C curve, R
PD
is
estimated to be approximately 40Ω. Assuming a 150pF
load capacitance, the fall time is about 13.2ns.
Although the outputs are considered to be “open-drain,”
they do have a weak pull-up capability (see COMPX or RST
Figure 6. Setting the Negative Adjustable Trip Point
–
+
2901 F06
V4
V
REF
13
12
V
TRIP
R4
1%
R3
1%
LTC2901
Table 2. Suggested 1% Resistor Values for the ADJ Inputs
V
SUPPLY
(V) V
TRIP
(V) R3 (kΩ) R4 (kΩ)
12 11.25 2150 100
10 9.4 1780 100
8 7.5 1400 100
7.5 7 1300 100
6 5.6 1020 100
5 4.725 845 100
3.3 3.055 511 100
3 2.82 464 100
2.5 2.325 365 100
1.8 1.685 237 100
1.5 1.410 182 100
1.2 1.120 124 100
1 0.933 86.6 100
0.9 0.840 68.1 100
Table 3. Suggested 1% Resistor Values for the –ADJ Input
V
SUPPLY
(V) V
TRIP
(V) R3 (kΩ) R4 (kΩ)
–2 –1.87 187 121
–5 –4.64 464 121
–5.2 –4.87 487 121
–10 –9.31 931 121
–12 –11.30 1130 121
Figure 5. Setting the Positive Adjustable Trip Point
–
+
+
–
0.5V
2901 F05
V3 OR V4
V
TRIP
R3
1%
R4
1%
LTC2901