Datasheet

LT3971A/LT3971A-5
20
3971af
APPLICATIONS INFORMATION
these components should be as small as possible. These
components, along with the inductor and output capacitor,
should be placed on the same side of the circuit board,
and their connections should be made on that layer. Place
a local, unbroken ground plane below these components.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB and R
T
nodes small so that the ground
traces will shield them 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 possible, and add thermal vias under and near
the LT3971A to additional ground planes within the circuit
board and on the bottom side.
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3971A circuits. However, these
capacitors can cause problems if the LT3971A is plugged
into a live supply. The low loss ceramic capacitor, combined
with stray inductance in series with the power source,
forms an under damped tank circuit, and the voltage at
the V
IN
pin of the LT3971A can ring to twice the nominal
input voltage, possibly exceeding the LT3971A’s rating and
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT3971A into an energized
supply, the input network should be designed to prevent
this overshoot. See Linear Technology Application Note
88 for a complete discussion.
High Temperature Considerations
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT3971A. 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 heat dissipated by the LT3971A. Placing
additional vias can reduce thermal resistance further. The
maximum load current should be derated as the ambient
temperature approaches the maximum junction rating.
Power dissipation within the LT3971A can be estimated by
calculating the total power loss from an efficiency measure-
ment and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
LT3971A power dissipation by the thermal resistance from
junction to ambient.
Also keep in mind that the leakage current of the power
Schottky diode goes up exponentially with junction tem-
perature. When the power switch is closed, the power
Schottky 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 input power. Therefore,
the catch Schottky diode must be selected with care to
avoid excessive increase in light load supply current at
high temperatures.
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.