Datasheet

LT1940/LT1940L
14
1940fa
Shorted Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, the LT1940 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 LT1940 is absent.
If the V
IN
and one of the RUN/SS pins are allowed to float,
then the LT1940’s internal circuitry will pull its quiescent
current through its SW pin. This is fine if your system can
tolerate a few mA of load in this state. With both RUN/SS
pins grounded, the LT1940 enters shutdown mode and
the SW pin current drops to ~30µA. However, if the V
IN
pin
is grounded while the output is held high, then parasitic
diodes inside the LT1940 can pull large currents from the
output through the SW pin and the V
IN
pin. A Schottky
diode in series with the input to the LT1940 will protect the
LT1940 and the system from a shorted or reversed input.
APPLICATIO S I FOR ATIO
WUU
U
Figure 5. Several Methods of Sequencing the Two Outputs. Channel 1 Starts First.
Figure 6. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output.
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board (PCB) layout. Figure 7
shows the high-di/dt paths in the buck regulator circuit.
Note that large, switched currents flow in the power
switch, the catch diode and the input capacitor. 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, and tie this ground plane to system
ground at one location, ideally at the ground terminal of the
output capacitor C2. Additionally, the SW and BOOST
nodes should be kept as small as possible. Figure 8 shows
recommended component placement with trace and via
locations.
OFF
1940 F05
RUN/SS1
PG1
ON
GND
OFF
RUN/SS1
ON
GND
RUN/SS2
OFF
RUN/SS1
ON
OFF2
ON2
GND
RUN/SS2
RUN/SS2
V
C2
PG1
1nF 1nF
1nF
2.2nF
1nF
1.5nF 1.5nF
(5b) Fewest Components
(5c) Independent Control of Channel 2
OFF
RUN/SS1
ON
GND
RUN/SS2
PG1
(5d) Doesn't Work !
(5a) Channel 2 is Delayed
V
IN
V
IN
V
OUT
SW
LT1940
D4
PARASITIC DIODE
1940 F06