LTC1421 Ver la hoja de datos (PDF) - Linear Technology
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LTC1421 Datasheet PDF : 24 Pages
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LTC1421/LTC1421-2.5
APPLICATIONS INFORMATION
Figure 8c shows how the comparator can be used to
generate a reset when the 12V supply (VOUTHI) drops
below 10.8V. The 5V supply (VOUTLO) also generates a
reset when it dips below 4.65V. When the 12V supply
drops below 10.8V, COMPOUT will pull the FB pin low
setting the internal threshold voltage for comparator 1 to
5.88V. Since VOUTLO is less than 5.88V, PWRGD immedi-
ately goes low and a reset is generated 32µs later.
Figure 8d shows how the comparator can be used to
override the internal reset voltage for a 5V supply on
VOUTLO.
VCCLO
6
VCCLO
20µA COMP1
20µA
7
RESET
TIMING
1.232V
3.3V
5V
20
LTC1421
73.5k
16
71.5k
26.7k
COMP2
11
10k
5%
15
107k
13
1%
38.3k
14
1%
8
1421 F08b
Figure 8b. Monitor 5V, Reset 3.3V at 2.9V
VCCLO
6
VCCLO
20µA COMP1
20µA
7
RESET
TIMING
5V
20
LTC1421
73.5k
16
71.5k
11
26.7k
15
COMP2
13
14
12V
107k
1%
13.7k
1%
1.232V
8
1421 F08c
Figure 8c. Reset 12V at 10.8V, Reset 5V at 4.65V
A 5.1k resistor is tied from the FB pin to VOUTLO, setting the
internal threshold to about 2.9V. The new reset threshold
voltage is set by the external resistive divider connected to
COMP2. When VOUTLO drops below the new threshold,
COMPOUT pulls FB to ground, changing the internal
threshold at COMP1 to 5.88V and generating a reset.
Finally, the comparator may be used to monitor a negative
supply as shown in Figure 8e. The external resistor divider
VCCLO
6
VCCLO
20µA COMP1
20µA
7
RESET
TIMING
1.232V
5V
20
5.1k
LTC1421
5%
73.5k
16 12V
71.5k
11
26.7k
15
COMP2
13
14
8
1421 F08d
102k
1%
38.3k
1%
Figure 8d. Reset 5V at 4.5V
5V
VCCLO
6
VCCLO
20µA COMP1
20µA
7
RESET
TIMING
20
LTC1421
73.5k
71.5k
16 12V
10k
5%
11
26.7k
15
COMP2
13
14
1.232V
8
1421 F08e
13.7k
1%
107k
1%
– 12V
Figure 8e. Monitor – 12V at – 10.8V, Reset 5V at 4.65V
10
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