LT3508 LINER [Linear Technology], LT3508 Datasheet - Page 18

• Manufacturer Part Number: LT3508
• Description: Dual Monolithic 1.4A Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

LT3508
applications or in battery back-up systems where a battery
or some other supply is diode OR-ed with the LT3508’s
output. If the V
is held high (either by a logic signal or because it is tied
to V
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 SHDN pin, the SW pin current will drop to essentially
zero. However, if the V
is held high, then parasitic diodes inside the LT3508 can
pull large currents from the output through the SW pin
and the V
only when the input voltage is present and that protects
against a shorted or reversed input.
APPLICATIONS INFORMATION
18
Figure 10. Diode D4 Prevents a Shorted Input from Discharging
a Backup Battery Tied to the Output
IN
), then the LT3508’s internal circuitry will pull its
V
IN
IN
PARASITIC DIODE
D4
pin. Figure 10 shows a circuit that will run
(11a) Example Layout for FE16 Package
IN
pin is allowed to fl oat and the SHDN pin
V
IN
IN
LT3508
pin is grounded while the output
Figure 11. A Good PCB Layout Ensures Proper Low EMI Operation
SW
3508 F10
V
OUT
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 11 shows the
recommended PCB layout with trace and via locations. Note
that large, switched currents fl ow in the LT3508’s V
SW pins, the catch diode (D1) and the input capacitor (C
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
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 LT3508 to additional ground planes
within the circuit board and on the bottom side.
(11b) Example Layout for QFN Package
C
nodes
IN
3508fb
and
IN
).