Datasheet

10
LTC2900
2900f
APPLICATIO S I FOR ATIO
WUUU
Figure 6. Setting the Negative Adjustable Trip Point
–
+
2900 F06
V4
V
REF
9
8
V
TRIP
R4
1%
R3
1%
LTC2900
sensed and ground, is connected to the high impedance
noninverting inputs (V3, V4). The trip voltage is calculated
from:
VV
R
R
TRIP
=+






05 1
3
4
.
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
V V No al
TRIP REF REF
=






=– ; . min
3
4
1 210
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.
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
2900 F05
V3 OR V4
V
TRIP
R3
1%
R4
1%
LTC2900