LT3480
18
3480fb
APPLICATIONS INFORMATION
source, forms an under damped tank circuit, and the
voltage at the V
IN
pin of the LT3480 can ring to twice the
nominal input voltage, possibly exceeding the LT3480’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3480 into an
energized supply, the input network should be designed
to prevent this overshoot. Figure 10 shows the waveforms
that result when an LT3480 circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The
fi rst plot is the response with a 4.7μF ceramic capacitor
at the input. The input voltage rings as high as 50V and
the input current peaks at 26A. A good solution is shown
in Figure 10b. A 0.7 resistor is added in series with the
input to eliminate the voltage overshoot (it also reduces
the peak input current). A 0.1μF capacitor improves high
frequency fi ltering. For high input voltages its impact on
effi ciency is minor, reducing effi ciency by 1.5 percent for
a 5V output at full load operating from 24V.
High Temperature Considerations
The PCB must provide heat sinking to keep the LT3480 cool.
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 LT3480. Place additional
vias can reduce thermal resistance further. With these steps,
the thermal resistance from die (or junction) to ambient can
be reduced to
JA
= 35°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 LT3480, it is possible
to dissipate enough heat to raise the junction temperature
beyond the absolute maximum of 125°C. When operating at
high ambient temperatures, the maximum load current should
be derated as the ambient temperature approaches 125°C.
Power dissipation within the LT3480 can be estimated by
calculating the total power loss from an effi ciency measure-
ment and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
LT3480 power dissipation by the thermal resistance from
junction to ambient.
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.
TYPICAL APPLICATIONS
SW
FB
V
C
PG
RT
V
IN
BD
V
IN
6.8V TO 36V
TRANSIENT
TO 60V*
V
OUT
5V
2A
4.7μF
0.47μF
22μF
100k
f = 800kHz
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB6R8M
D
16.2k
40.2k
L
6.8μH
536k
GND
470pF
ON OFF
LT3480
3480 TA02
RUN/SS
BOOST
SYNC
5V Step-Down Converter
