LTC4420
applicaTions inForMaTion
V2 Monitoring and Control
V2 monitoring duration and time between monitoring
events are set by input V2TEST, V1 validity and V2 validity
asdeterminedpreviously. Completebehaviorisdescribed
by the state diagram shown in Figure 4. This implementa-
tion was chosen for the following reasons,
The LTC4420 monitors V2 voltage through an external
resistivedividerconnectedbetweenV2,V2UVandGNDSW.
When V2 is being monitored, open-drain output GNDSW
is pulled low to activate the resistive divider, otherwise
it is released high. V2UV is monitored by comparator
CV2UV, whose output is latched at the end of the moni-
toring period. This latched output establishes V2 validity
and is used in Table 1.
1. To provide flexibility in monitoring and disconnect-
ing the backup battery as required by the application,
while minimizing current draw through the V2 resistive
divider. V2TEST and V2DIS need to be actively driven
to achieve this.
POWER
UP
THERMAL
SHUTDOWN
OR V2 < 1.55V OR
SWITCHOVER
TO V2
EXIT THERMAL
SHUTDOWN
OR
DISABLE
V2 MONITORING
V2 NOT
VALID
V2TEST
HIGH
AND
AND V1 VALID
2ms
128ms
V2 MONITORING
V2 MONITORING
WHEN V1 IS
VALID
V2
VALID
V2TEST
HIGH
AND
V1
NOT VALID
V2TEST
LOW
V2
NOT VALID
AND
OR
(
)
2ms
WAIT FOR RISING
V2TEST OR
OVERRIDE CONDITION
V2 MONITORING
WHEN V1 IS
INVALID
V1 VALID AND V2TEST LOW
V1 NOT VALID AND (V2TEST LOW AND V2 NOT VALID)
V2 VALID OR V2TEST HIGH
4420 F04
Figure 4. State Diagram Describing V2 Monitoring
4420f
12
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Linear Systems [ Linear Systems ]
