LT3990/LT3990-3.3/LT3990-5
16
3990fa
APPLICATIONS INFORMATION
Hot Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3990 circuits. However, these ca-
pacitors can cause problems if the LT3990 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 LT3990 can ring to twice the nominal
input voltage, possibly exceeding the LT3990’s rating and
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT3990 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
LT3990. The exposed pad on the bottom 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 LT3990. 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 LT3990 can be estimated by
calculating the total power loss from an efficiency measure-
ment and subtracting inductor loss. The die temperature
is calculated by multiplying the LT3990 power dissipation
by the thermal resistance from junction to ambient.
Finally, be aware that at high ambient temperatures the
internal Schottky diode will have significant leakage current
(see Typical Performance Characteristics) increasing the
quiescent current of the LT3990 converter.
Fault Tolerance
The LT3990 regulator in the MSOP package is designed to
tolerate single fault conditions. Shorting any two adjacent
pins together or leaving any one single pin floating does
not raise V
OUT
above the programmed value or cause
damage to the part.
The NC pins are not connected to internal circuitry and
must be left floating to ensure fault tolerance.
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 100
shows how to generate a bipolar output supply using a
buck regulator.
