lt3695 Linear Technology Corporation, lt3695 Datasheet - Page 21

• Manufacturer Part Number: lt3695
• Description: 1a Fault Tolerant Micropower Step-down Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3695’s output. If the V
RUN/SS pin is held high (either by a logic signal or because
it is tied to V
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
essentially zero. However, if the V
the output is held high, then parasitic diodes inside the
LT3695 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 9 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
fl ow in the LT3695’s V
diode and the input capacitor (C
these components should be as small as possible. These
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
IN
MBRS140
D4
IN
), then the LT3695’s internal circuitry will
V
RUN/SS
V
IN
C
GND
LT3695
IN
PGND
BD
IN
pin. Figure 8 shows a circuit that
IN
, SW and PGND pins, the catch
BOOST
FB
pin is allowed to fl oat and the
SW
DA
IN
IN
). The loop formed by
pin is grounded while
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BACKUP
V
OUT
components, along with the inductor and output capacitor
(C
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
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 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.
OUT
GND
), should be placed on the same side of the circuit
Figure 9. A Good PCB Layout Ensures Proper,
Low EMI Operation
R
V
T
T
IN
C2
D1
and V
C
C1
nodes small so that the ground
V
OUT
L
GND
R
C
C3
R2
R1
C
C
LT3695
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3695f