Datasheet

LT3692A
26
3692afc
For more information www.linear.com/3692A
applicaTions inForMaTion
Shorted and Reverse Input Protection
If the inductor is chosen so that it won’t saturate excessively,
an LT3692A step-down regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3692A is absent. This may occur in battery charging
applications or in battery back-up systems where a battery
or some other supply is diode OR-ed with the LT3692A’s
output. If the V
IN1/2
pin is allowed to float and the SHDN
pin is held high (either by a logic signal or because it is
tied to V
IN
), then the LT3692A’s internal circuitry will pull
its quiescent current through its SW pin. This is fine if your
system can tolerate a few mA in this state. If you ground
the SHDN pin, the SW pin current will drop to essentially
zero. However, if the V
IN
pin is grounded while the output
is held high, then parasitic diodes inside the LT3692A can
pull large currents from the output through the SW pin
and the V
IN1/2
pin. Figure 17 shows a circuit that will run
only when the input voltage is present and that protects
against a shorted or reversed input.
V
IN
V
IN1/2
V
OUT1/2
SW
LT3692A
D4
PARASITIC DIODE
3692a F17
Figure 17. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output
Figure 18. Subtracting the Current When the Switch Is On (18a)
from the Current When the Switch Is Off (18b) Reveals the Path of
the High Frequency Switching Current (18c). Keep this Loop Small.
The Voltage on the SW and BST Traces Will Also Be Switched; Keep
These Traces as Short as Possible. Finally, Make Sure the Circuit
Is Shielded with a Local Ground Plane
V
IN
GND
(18a)
LT3692A
SW
V
IN
GND
(18c)
LT3692A
SW
3692a F18
V
IN
GND
(18b)
LT3692A
SW