Datasheet

11
LTC2902
2902f
APPLICATIO S I FOR ATIO
WUUU
In the negative adjustable mode, the noninverting input on
the V4 comparator is connected to ground (Figure 6). The
tap point on an external resistive divider, connected be-
tween the negative voltage being sensed and the V
REF
pin,
is connected to the high impedance inverting input (V4).
V
REF
provides the necessary level shift required to operate
at ground. The trip voltage is calculated from:
VV
R
R
VV
TRIP REF REF
=






=–;.
3
4
1 210
T0,T1 Low (5% Tolerance Mode)
Once the resistor divider is set in the 5% tolerance mode,
there is no need to change the divider for the other
tolerance modes (7.5%, 10%, 12.5%) because V
REF
is
scaled accordingly, moving the trip point in –2.5%
increments.
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.
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 chip. 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) have strong pull-down
capability. If the external load capacitance (C
LOAD
) for a
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
Figure 6. Setting the Negative Adjustable Trip Point
–
+
2902 F06
V4
V
REF
13
12
V
TRIP
R4
1%
R3
1%
LTC2902
–
+
+
–
0.5V
5% TOLERANCE MODE
2902 F05
V3 OR V4
V
TRIP
R3
1%
R4
1%
LTC2902