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 = 1.21k.
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 (k) R4 (k)
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 (k) R4 (k)
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