Datasheet

LT3990/LT3990-3.3/LT3990-5
15
3990fa
APPLICATIONS INFORMATION
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, a LT3990 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
LT3990 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode ORed with the LT3990’s
output. If the V
IN
pin is allowed to float and the EN/UVLO
pin is held high (either by a logic signal or because it is
tied to V
IN
), then the LT3990’s internal circuitry will pull
its quiescent current through its SW pin. This is fine if the
system can tolerate a few µA in this state. If the EN/UVLO
pin is grounded, the SW pin current will drop to 0.7µA.
However, if the V
IN
pin is grounded while the output is held
high, regardless of EN/UVLO, parasitic diodes inside the
LT3990 can pull current 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.
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 flow in the LT3990’s V
IN
and SW pins,
the internal catch diode and the input capacitor. The loop
formed by these components should be as small as pos-
sible. These components, along with the inductor and
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 LT3990 Runs Only when the Input Is Present
Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation
Figure 6. Undervoltage Lockoout
+
–
1.19V
SHDN
3990 F06
LT3990
EN/UVLO
V
IN
V
IN
R3
R4
BD
LT3990
BOOSTV
IN
EN/UVLO
V
IN
V
OUT
BACKUP
3990 F07
SW
D4
FBGND
+
6
8
7
9
10
5
4
2
3
1
VIAS TO LOCAL GROUND PLANE
VIAS TO V
OUT
EN/UVLO
GND
GND
PG
V
OUT
GND
V
IN
3990 F08
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 nodes small so
that the ground traces will shield them from the SW and
BOOST nodes. The exposed pad on the bottom 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 LT3990 to additional ground planes within the circuit
board and on the bottom side.