LT3695 Series
21
3695fa
then the LT3695 regulators’ 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 essen-
tially zero. However, if the V
IN
pin is grounded while the
output is held high, then parasitic diodes inside the LT3695
regulators can pull large currents from the output through
the SW pin and the V
IN
pin. Figure 8 shows a circuit that
will run only when the input voltage is present and that
protects against a shorted or reversed input.
APPLICATIONS INFORMATION
3695 F09
LT3695
V
IN
V
C
BACKUP
D4
MBRS140
V
OUT
V
IN
BD
GND
SW
DA
FB
RUN/SS
BOOST
PGND
Figure 8. 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 Regulator Runs Only When the Input
Is Present
V
OUT
GND
C2
D1
R
T
R
C
C1
C3
L
V
IN
GND
3695 F09
R1
R2
C
C
Figure 9. A Good PCB Layout Ensures Proper,
Low EMI Operation (LT3695)
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figures 9 and
10 show the recommended component placement with
trace, ground plane and via locations. Note that large,
switched currents fl ow in the LT3695 regulators’ V
IN
, SW
and PGND pins, the catch diode and the input capacitor
(C
IN
). The loop formed by these components should be
as small as possible. These components, along with the
inductor and output capacitor (C
OUT
), should be placed on
the same side of the circuit board, and their connections
should be made on that layer. All connections to GND should
be made at a common star ground point or directly to a
local, unbroken ground plane below these components.
The SW and BOOST nodes should be laid out carefully to
avoid interference. Finally, keep the FB, R
T
and V
C
nodes
small so that the ground traces will shield them from the
SW and BOOST nodes. To keep thermal resistance low,
extend the ground plane as much as possible and add
thermal vias under and near the LT3695 regulators to any
additional ground planes within the circuit board and on the
bottom side. Keep in mind that the thermal design must
keep the junctions of the IC below the specifi ed absolute
maximum temperature.
Figure 10. A Good PCB Layout Ensures Proper,
Low EMI Operation (LT3695-3.3, LT3695-5)
V
OUT
GND
C2
D1
R
T
R
C
C1
C3
L
V
IN
GND
3695 F10
C
C
