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LT3695HMSE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30
LT3695 Series
22
3695fa
APPLICATIONS INFORMATION
High Temperature Considerations
The PCB must provide heat sinking to keep the LT3695
regulators cool. The exposed pad on the bottom of the
package may be soldered to a copper area which should be
tied to large copper layers below with thermal vias; these
layers will spread the heat dissipated by the LT3695 regu-
lators. Place additional vias to reduce thermal resistance
further. With these steps, the thermal resistance from die
(or junction) to ambient can be reduced to θ
JA
= 40°C/W
or less. With 100 LFPM airfl ow, this resistance can fall
by another 25%. Further increases in airfl ow will lead
to lower thermal resistance. Because of the large output
current capability of the LT3695 regulators, it is possible
to dissipate enough heat to raise the junction temperature
beyond the absolute maximum. When operating at high
ambient temperatures, the maximum load current should
be derated as the ambient temperature approaches these
maximums. If the junction temperature reaches the ther-
mal shutdown threshold, the parts will stop switching to
prevent internal damage due to overheating.
Power dissipation within the LT3695 regulators can be esti-
mated by calculating the total power loss from an effi ciency
measurement. The die temperature rise is calculated by
multiplying the power dissipation of the LT3695 regula-
tors by the thermal resistance from junction to ambient.
Die temperature rise was measured on a 2-layer, 10cm ×
10cm circuit board in still air at a load current of 1A (f
SW
=
800kHz). For a 12V input to 5V output the die temperature
elevation above ambient was 22°C with the exposed pad
soldered and 44°C without the exposed pad soldered.
Fault Tolerance
The LT3695 regulators are designed to tolerate single fault
conditions. Shorting two adjacent pins together or leaving
one single pin fl oating does not raise V
OUT
or cause damage
to the LT3695 regulators. However, the application circuit
must meet the requirements discussed in this section in
order to achieve this tolerance level.
Tables 5 and 6 show the effects that result from shorting
adjacent pins or from a fl oating pin, respectively.
For the best fault tolerance to inadvertent adjacent pin
shorts, the RUN/SS pin must not be directly connected to
either ground or V
IN
. If there was a short between RUN/SS
and SW then connecting RUN/SS to V
IN
would tie SW
to V
IN
and would thus raise V
OUT
. Likewise, grounding
RUN/SS would tie SW to ground and would damage the
power switch if this is done when the power switch is on.
A short between RT and a RUN/SS pin that is connected
to V
IN
would violate the absolute maximum ratings of the
RT pin. Therefore, the current supplying the RUN/SS pin
must be limited, for example, with resistor R3 in Figures
11 and 12. In case of a short between RUN/SS and SW this
resistor charges C2 through the inductor L1 if the current
it supplies from V
IN
is not completely drawn by R
LOAD
, R1
BDV
IN
LT3695
RUN/SS
R3
RT
V
IN
C3
L1
D1
D2
R2
R1
C2
V
OUT
C
SS
220nF
3695 F11
BOOST
SW
DA
FB
R
SS
47
R
T
R
LOAD
Figure 11. LT3695: The Dashed Lines Show Where a Connection
Would Occur if There Were an Inadvertent Short from RUN/SS
to an Adjacent Pin or from BOOST to BD. In These Cases, R3
Protects Circuitry Tied to the RT or SW Pins, and D2 Shields
BOOST from V
OUT
. If C
SS
Is Used for Soft Start, R
SS
Isolates It
from SW
Figure 12. LT3695-3.3, LT3695-5: The Dashed Lines Show
Where a Connection Would Occur if There Were an Inadvertent
Short from RUN/SS to an Adjacent Pin. In These Cases, R3
Protects Circuitry Tied to the RT or SW Pins. R4 Provides an
Additional Load and May Be Necessary in Certain Situations
(See Text). If C
SS
Is Used for Soft Start, R
SS
Isolates It from SW
BOOST
V
IN
LT3695-3.3
LT3695-5
RUN/SS
R3
RT
V
IN
C3
L1
D1
R4
C2
V
OUT
C
SS
220nF
3695 F12
SW
DA
OUT1
OUT2
R
SS
47
R
T
R
LOAD
LT3695 Series
23
3695fa
APPLICATIONS INFORMATION
Table 5: Effects of Pin Shorts
PINS EFFECT
PGND-DA No effect if V
IN
< V
IN(MAX)
. See Input Voltage Range section for description of V
IN(MAX)
.
SW-RUN/SS The result of this short depends on the load resistance and on R3 (Figure 10). See the following discussion.
RUN/SS-RT No effect or V
OUT
will fall below regulation voltage if I
R3
(Figure 10) < 1mA.
RT
-SYNC No effect or V
OUT
will fall below regulation voltage if the current into the RT pin is less than 1mA.
SYNC-V
IN
No effect if V
IN
does not exceed the absolute maximum voltage of SYNC (20V).
PG-GND No effect.
GND-BD (LT3695) V
OUT
may fall below regulation voltage, power dissipation of the power switch will be increased. Note that this short also
grounds the voltage source supplying BD. Make sure it is safe to short the supply for BD to ground. For this reason BD should
not be connected to V
IN
, but it is safe to connect it to V
OUT
.
BD-BOOST (LT3695) If diode D2 (see Figure 10) is used, no effect or V
OUT
may fall below regulation voltage. Otherwise the device may be damaged.
GND-OUT2
(LT3695-3.3, LT3695-5)
V
OUT
will fall below regulation voltage, because this shorts the output to ground. As a result, the power dissipation of the part
may increase.
Table 6: Effects of Floating Pins
PIN EFFECT
PGND No effect if the Exposed Pad is soldered.
Otherwise: V
OUT
may fall below regulation voltage. Make sure that V
IN
< V
IN(MAX)
(see Input Voltage Range section for details)
and provide a bypass resistor at the DA pin. See the following discussion.
DA V
OUT
may fall below regulation voltage. Make sure that V
IN
< V
IN(MAX)
(see Input Voltage Range section for details) and provide
a bypass resistor. See the following discussion.
SW V
OUT
will fall below regulation voltage.
RUN/SS V
OUT
will fall below regulation voltage.
RT V
OUT
will fall below regulation voltage.
SYNC V
OUT
may fall below regulation voltage. A fl oating SYNC pin confi gures the LT3695 for pulse-skipping mode. However, a
fl oating SYNC pin is sensitive to noise which can degrade device performance.
V
IN
V
OUT
will fall below regulation voltage.
V
C
V
OUT
may fall below regulation voltage. Disconnecting the V
C
pin alters the loop compensation and potentially degrades device
performance. The output voltage ripple will increase if the part becomes unstable.
FB (LT3695) V
OUT
will fall below regulation voltage.
PG No effect.
GND Output maintains regulation, but potential degradation of device performance.
BD (LT3695) V
OUT
may fall below regulation voltage. If BD is not connected, the boost capacitor cannot be charged and thus the power
switch cannot saturate properly, which increases its power dissipation.
OUT1, OUT2
(LT3695-3.3, LT3695-5)
No effect.
BOOST V
OUT
may fall below regulation voltage. If BOOST is not connected, the boost capacitor cannot be charged and thus the power
switch cannot saturate properly, which increases its power dissipation.
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
VV
µA
IN MIN
3
25
75
≤
()
–.
.
The current through R3 is maximal at V
IN(MAX)
with RUN/SS
shorted to SW:
I
VV
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
RRR
I
R
OUT
LO AD
BD3
12
≤
+
()
+
||
Without load (R
LOAD
= ∞) and assuming the minimum
current of 35µA into the BD pin, this leads to
RR
V
VV
R
µA
OUT
IN MAX OUT
12
3
35
+ ≤
()
–
–
as upper limit for the feedback resistors. For V
OUT
< 2.5V
assume no current drawn by the BD pin, which gives
RR
VR
VV
OUT
IN MAX OUT
12
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
RR
I
R
OUT
LO AD
OUT312
4
≤ +
||
,
Without load (R
LOAD
= ∞) and assuming the minimum
current of 43A into the OUT1,2 pins, this leads to:
R
V
VV
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
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30

LT3695HMSE#PBF

Mfr. #:
Buy LT3695HMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 36V (60V Transient), 1A (Iout) MicroPower 2.2MHz Step-Down Switching Regulator with 1A Fault Protection in MSOP-16E
Lifecycle:
New from this manufacturer.
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