LT3975
APPLICATIONS INFORMATION
D4
High Temperature Considerations
B360A
V
V
BOOST
IN
IN
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT3975. The exposed pad on the bottom of the package
must be soldered to a ground plane. This ground should
be tied to large copper layers below with thermal vias;
these layers will spread the heat dissipated by the LT3975.
Placing additional vias can reduce the thermal resistance
further. Whenoperatingathighambienttemperatures, the
maximum load current should be derated as the ambient
temperature approaches the maximum junction rating.
V
EN
SW
OUT
LT3975
OUT
FB
+
GND
BACKUP
3975 F09
Figure 9. 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 LT3975 Runs Only When the Input
Is Present
Power dissipation within the LT3975 can be estimated by
calculatingthetotalpowerlossfromanefficiencymeasure-
ment and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
LT3975 power dissipation by the thermal resistance from
junction to ambient.
SS
SYNC
Also keep in mind that the leakage current of the power
Schottky diode goes up exponentially with junction tem-
perature.Whenthepowerswitchisoff,thepowerSchottky
diode is in parallel with the power converter’s output
filter stage. As a result, an increase in a diode’s leakage
current results in an effective increase in the load, and a
corresponding increase in the input quiescent current.
Therefore, the catch Schottky diode must be selected
with care to avoid excessive increase in light load supply
current at high temperatures.
V
OUT
17
V
OUT
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
• • •
FB
BST
PG
RT
OUT
EN
V
IN
SW
3975 F10
Figure 10. Layout Showing a Good PCB Design
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.
unbrokengroundplanebelowthesecomponents. TheSW
and BOOST nodes should be as small as possible. Finally,
keep the FB and RT nodes small so that the ground traces
will shield it from the SW and BOOST nodes. The exposed
pad on the bottom of the package must be soldered to
ground so that the pad acts as a heat sink. To keep thermal
resistance low, extend the ground plane as much as pos-
sible, and add thermal vias under and near the LT3975 to
additional ground planes within the circuit board and on
the bottom side.
3975f
21
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