lt3685emse-trpbf Linear Technology Corporation, lt3685emse-trpbf Datasheet - Page 16

• Manufacturer Part Number: lt3685emse-trpbf
• Description: 38v, 2a, 2.4mhz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3685
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3685 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
LT3685 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 LT3685’s
output. If the V
pin is held high (either by a logic signal or because it is
tied to V
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
the output is held high, then parasitic diodes inside the
LT3685 can pull large currents from the output through
the SW pin and the V
will run only when the input voltage is present and that
protects against a shorted or reversed input.
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 LT3685’s V
diode (D1) and the input capacitor (C1). The loop formed
16
V
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 LT3685
Runs Only When the Input is Present
IN
MBRS140
IN
D4
), then the LT3685’s internal circuitry will pull
IN
V
RUN/SS
V
IN
C
pin is allowed to fl oat and the RUN/SS
LT3685
IN
GND FB
BOOST
pin. Figure 7 shows a circuit that
SW
IN
IN
and SW pins, the catch
pin is grounded while
3685 F07
V
BACKUP
OUT
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
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 LT3685 to additional ground planes within the circuit
board and on the bottom side.
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3685 circuits. However, these capaci-
tors can cause problems if the LT3685 is plugged into a
Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation
VIAS TO LOCAL GROUND PLANE
VIAS TO V
D1
OUT
L1
C1
VIAS TO SYNC
V
OUT
C
nodes small so that the ground
GND
VIAS TO RUN/SS
VIAS TO PG
C2
R
R
PG
RT
VIAS TO V
OUTLINE OF LOCAL
GROUND PLANE
C
R1
C
R
C
R2
IN
3685 F08
3685f