Datasheet

LTC2938/LTC2939
12
293839ff
APPLICATIONS INFORMATION
Figure 3. Setting the Positive Adjustable Trip Point
Using the Adjustable Thresholds
The reference inputs on the V3 and/or V4 comparators are
set to 0.5V when the positive adjustable modes are se-
lected (Figure 3). The LTC2939 V5 and V6 comparators are
always in positive adjustable mode with a 0.5V reference.
The tap point on an external resistive divider, connected
between the positive voltage being sensed and ground, is
connected to the high impedance, adjustable inputs (V3,
V4, V5 and V6). Calculate the trip voltage from:
VV
R
R
TRIP
=+
⎛
⎝
⎜
⎞
⎠
⎟
05 1
3
4
.•
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:
1 210
1
1 210
.
.
V
mA
k=
Tables 2 and 3 offer suggested 1% resistor values for
various adjustable applications assuming 5% monitor
thresholds.
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 Negative
ADJ Inputs
V
SUPPLY
(V) V
TRIP
(V) R3 (kΩ) R4 (kΩ)
–2 –1.87 1.87 121
–5 –4.64 464 121
–5.2 –4.87 487 121
–10 –9.31 931 121
–12 –11.30 1130 121
Although all of the 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 supply for the device. Filter capacitors on the V3, V4,
V5 and V6 inputs are allowed.
In the negative adjustable mode, the high impedance,
adjustable input on the V4 comparator is connected to
ground (Figure 4). The tap point on an external resistive
divider, connected between the negative voltage being
sensed and the V
REF
output, is connected to the high
impedance adjustable input (V4). V
REF
provides the nec-
essary level shift required to operate at ground. The nega-
tive trip voltage is calculated from:
VV
R
R
VVNoal
TRIP REF REF
= − =•; . min
3
4
1 210
Figure 4. Setting the Negative Adjustable Trip Point
+
–
0.5V
293839 F03
V3, V4, V5 OR V6
V
TRIP
R3
1%
R4
1%
LTC2938/LTC2939
293839 F04
V4
V
REF
V
TRIP
R4
1%
R3
1%
LTC2938/LTC2839