lt3695 Linear Technology Corporation, lt3695 Datasheet - Page 24

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

APPLICATIONS INFORMATION
LT3695
V
pin (if connected to V
the LT3695 stops switching. The resistive divider formed
by R3, R
to exceed its nominal value at the required maximum input
voltage V
RUN/SS at the required minimum input voltage V
for normal non-fault situations. Based on the maximum
RUN/SS current of 7.5μA at V
The current through R3 is maximal at V
shorted to SW:
This current must be drawn by R
BD pin, if connected to V
Without load (R
current of 35μA into the BD pin, this leads to
as upper limit for the feedback resistors. For V
assume no current drawn by the BD pin, which gives
24
IN
R
I
I
R
R
R
R
is not completely drawn by R
3
1
1
3
3
+
+
≤
=
≤
R
R
LOAD
V
R
V
2
2
IN(MAX)
IN MIN
IN MAX
LOAD
≤
≤
(
(
7 5
V
, and R1 + R2 must be adjusted for V
V
.
IN MAX
IN MAX
V
R
||
)
(
µA
OUT
(
V
)
LOAD
. R3 must supply suffi cient current into
– .
3
(
OUT
–
R
2 5
1
V
R
+
OUT
)
)
OUT
3
•
V
= ∞) and assuming the minimum
–
R
–
V
R
V
2
OUT
V
3
). Since this causes V
OUT
OUT
)
OUT
+
:
I
BD
RUN/SS
–
LOAD
35
LOAD
µA
, R1 + R2, and the BD
IN(MAX)
= 2.5V this gives
, R1 + R2, and the
with RUN/SS
OUT
OUT
OUT
to rise,
IN(MIN)
< 2.5V
not
Table 7 shows example values for common applications.
R
have to charge C
shorted, which may damage the power switch.
If RUN/SS is controlled by an external circuitry, the current
this circuitry can supply must be limited. This can be done
as discussed above. In addition, it may be necessary to
protect this external circuitry from the voltage at SW, for
example by using a diode.
Table 7. Example Values for R1, R2 and R3 for Common
Combinations of V
R1 + R2 in Normal Operation
The BOOST pin must not be shorted to a low impedance
node like V
ance, 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
V
SS
IN(MAX)
(V)
16
36
16
36
16
36
16
36
16
36
27
36
must be included as the switch node would otherwise
V
IN(MIN)
OUT
(V)
3.8
3.8
4.5
4.5
10
10
14
14
5
5
7
7
that clamps its voltage. For best fault toler-
IN
SS
and V
V
(V)
1.8
1.8
2.5
2.5
3.3
3.3
12
12
if the SW pin and the RUN/SS pin are
OUT
5
5
8
8
OUT
. I
(kΩ)
169
169
261
261
365
365
274
590
200
475
301
442
R3
R1+R2
is the Current Drawn by
(kΩ)
11.5
4.75
93.1
16.9
43.2
432
536
221
562
280
511
511
R1
(kΩ)
9.09
3.74
43.2
7.87
13.7
42.2
61.9
30.9
36.5
36.5
137
102
R2
I
R1+R2
(μA)
212
101
87
18
58
19
13
26
22
22
6
8
3695f