LT3695 Series
24
3695fa
+ R2, and the BD pin (if connected to V
OUT
) in the case of
the LT3695, or by R
LOAD
, R4 and the OUT1,2 pins in the
case of the LT3695-3.3 and LT3695-5. Since this causes
V
OUT
to rise, the LT3695 regulators stop switching. The
resistive divider formed by R3, R
LOAD
and R1 + R2 and
R4, respectively, must be adjusted for V
OUT
not to exceed
its nominal value at the required maximum input voltage
V
IN(MAX)
. R3 must supply suffi cient current into RUN/SS
at the required minimum input voltage V
IN(MIN)
for normal
non-fault situations. Based on the maximum RUN/SS cur-
rent of 7.5μA at V
RUN/SS
= 2.5V this gives
R
V
V
µA
IN MIN
3
2 5
7 5
≤
(
)
– .
.
The current through R3 is maximal at V
IN(MAX)
with RUN/SS
shorted to SW:
I
V
V
R
R
IN MAX
OUT
3
3
=
(
)
–
For the LT3695, this current must be drawn by R
LOAD
,
R1 + R2, and the BD pin, if connected to V
OUT
:
I
V
R
R
R
I
R
OUT
LOAD
BD
3
1
2
≤
+
(
)
+
||
Without load (R
LOAD
= ∞) and assuming the minimum
current of 35μA into the BD pin, this leads to
R
R
V
V
V
R
µA
OUT
IN MAX
OUT
1
2
3
35
+
≤
(
)
–
–
as upper limit for the feedback resistors. For V
OUT
< 2.5V
assume no current drawn by the BD pin, which gives
R
R
V
R
V
V
OUT
IN MAX
OUT
1
2
3
+
≤
•
–
(
)
For the LT3695-3.3 and LT3695-5, the current through R3
must be drawn by R
LOAD
, R4 and the OUT1,2 pins:
I
V
R
R
I
R
OUT
LOAD
OUT
3
12
4
≤
+
||
,
Without load (R
LOAD
= ∞) and assuming the minimum
current of 43μA into the OUT1,2 pins, this leads to:
R
V
V
V
R
µA
OUT
IN MAX
OUT
4
3
43
≤
(
)
–
–
as upper limit for R4. Depending on the required input
voltage range, R4 may be omitted.
Tables 7 and 8 show example values for common appli-
cations. R
SS
must be included as the switch node would
otherwise have to charge C
SS
if the SW pin and the RUN/SS
pin are 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. LT3695: Example Values for R1, R2 and R3 for Common
Combinations of V
IN
and V
OUT
. I
R1+R2
is the Current Drawn by
R1 + R2 in Normal Operation
V
IN(MAX)
(V)
V
IN(MIN)
(V)
V
OUT
(V)
R3
(kΩ)
R1
(kΩ)
R2
(kΩ)
I
R1+R2
(μA)
16
3.8
1.8
169
11.5
9.09
87
36
3.8
1.8
169
4.75
3.74
212
16
4.5
2.5
261
93.1
43.2
18
36
4.5
2.5
261
16.9
7.87
101
16
5.3
3.3
365
432
137
6
36
5.3
3.3
365
43.2
13.7
58
16
7
5
274
536
102
8
36
7
5
590
221
42.2
19
16
10
8
200
562
61.9
13
36
10
8
475
280
30.9
26
27
14
12
301
511
36.5
22
36
14
12
442
511
36.5
22
APPLICATIONS INFORMATION
