LT3693
16
3693f
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3693 buck 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
LT3693 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3693’s
output. If the V
IN
pin is allowed to fl oat and the RUN/SS
pin is held high (either by a logic signal or because it is
tied to V
IN
), then the LT3693’s internal circuitry will pull
its quiescent current through its SW pin. This is fi ne if
your system can tolerate a few mA in this state. If you
ground the RUN/SS 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
LT3693 can pull large currents from the output through
the SW pin and the V
IN
pin. Figure 7 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
Figure 7. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output. It Also
Protects the Circuit from a Reversed Input. The LT3693
Runs Only When the Input is Present
V
IN
BOOST
GND FB
RUN/SS
V
C
SW
D4
MBRS140
V
IN
LT3693
3693 F08
V
OUT
BACKUP
VIAS TO LOCAL GROUND PLANE
VIAS TO V
OUT
VIAS TO RUN/SS
VIAS TO PG
VIAS TO V
IN
OUTLINE OF LOCAL
GROUND PLANE
3693 F09
L1
C2
R
RT
R
PG
R
C
R2
R1
C
C
V
OUT
D1
C1
GND
VIAS TO SYNC
Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation
APPLICATIONS INFORMATION
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 8 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents fl ow in the LT3693’s V
IN
and SW pins, the catch
diode (D1) and the input capacitor (C1). The loop formed
by these components should be as small as possible. These
components, along with the inductor and output capacitor,
should be placed on the same side of the circuit board,
and their connections should be made on that layer. Place
a local, unbroken ground plane below these components.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB and V
C
nodes small so that the ground
traces will shield them from the SW and BOOST nodes.
The Exposed Pad on the bottom of the package must be
soldered to ground so that the pad acts as a heat sink. To
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near
the LT3693 to additional ground planes within the circuit
board and on the bottom side.
