LT3695 Series
25
3695fa
APPLICATIONS INFORMATION
Table 8. LT3695-3.3, LT3695-5: Example Values for R3 and R4
for Common Combinations of V
IN
and V
OUT
. I
R4
is the Current
Drawn by R4 in Normal Operation
V
IN(MAX)
(V)
V
IN(MIN)
(V)
V
OUT
(V)
R3
(kΩ)
R4
(kΩ)
I
R4
(μA)
16 5.3 3.3 309 None
24 5.3 3.3 365 215 15
36 5.3 3.3 365 66.5 50
16 7 5 267 None
24 7 5 464 None
36 7 5 590 442 11
The BOOST pin must not be shorted to a low impedance
node like V
OUT
that clamps its voltage. For best fault toler-
ance of the LT3695, supply current into the BD pin through
the Schottky diode D2 as shown in Figure 10. Note that
this diode must be able to handle the maximum output
current in case there is a short between the BD pin and
the GND pin.
A short between RUN/SS and SW may also increase the
output ripple. To suppress this, connect the soft-start
network consisting of R
SS
and C
SS
to RUN/SS as shown
in Figure 10. C
SS
should not be smaller than 0.22µF.
The SYNC pin must not be directly connected to either
ground or V
IN
. A short between RT and a SYNC pin that
is connected to V
IN
could violate the absolute maximum
ratings of the RT pin. A short between the SYNC pin and
the V
IN
pin could damage an external driver circuit which
may be connected to SYNC or would short V
IN
to ground
if SYNC is grounded.
V
IN
LT3695
LT3695-3.3
LT3695-5
C
S
100pF
R
S
100k
SYNCSYNC
RT
V
IN
3695 F13
R
T
The recommended connection for SYNC is shown in
Figure 13. If SYNC is to be driven by an external circuitry,
R
S
may be used to isolate this circuitry from V
IN
. C
S
must
be used in this case to provide a low impedance path
for the synchronization signal. If SYNC is pulled low, R
S
prevents V
IN
from being shorted to ground in case of
an inadvertent short between SYNC and V
IN
. If SYNC is
pulled high to V
IN
, then R
S
protects the RT pin during an
inadvertent short between SYNC and RT.
If the DA pin or the PGND pin are inadvertently left fl oat-
ing, the current path of the catch diode is interrupted
unless a bypass resistor is connected from DA to ground.
Use a 360m (5% tolerance) resistor rated for a power
dissipation of:
P = I
2
LOAD(MAX)
• 0.36 • (1 – DC
MIN
)
where I
LOAD(MAX)
is the maximum load current and DC
MIN
is the minimum duty cycle. For example, this would require
a power rating of at least 219mW for an output current of
800mA and a minimum duty cycle of 5%. Make sure not
to exceed V
IN(MAX)
(see Input Voltage Range section for
details) during start-up or overload conditions.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 318
shows how to generate a bipolar output supply using a
buck regulator.
Figure 13. The Dashed Lines Show Where a Connection Would Occur
if There Were an Inadvertent Short from SYNC to an Adjacent Pin. In
This Case, R
S
Protects Circuitry Connecting to SYNC
