Datasheet

LT3481
18
3481f
TYPICAL APPLICATIONS
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 0.7Ω 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 LT3481
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 lay-
ers will spread the heat dissipated by the LT3481. Place
additional vias can reduce thermal resistance further. With
these steps, the thermal resistance from die (or junction)
to ambient can be reduced to θ
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 re-
sistance. Because of the large output current capability of
the LT3481, 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 LT3481 can be estimated
by calculating the total power loss from an effi ciency
measurement and subtracting the catch diode loss. The
die temperature is calculated by multiplying the LT3481
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.
APPLICATIONS INFORMATION
5V Step-Down Converter
SW
BIAS
FB
V
C
PG
RT
V
IN
BD
V
IN
6.3V TO 34V
V
OUT
5V
2A
4.7µF
0.47µF
22µF
200k
f = 800kHz
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB6R8M
D
20k
60.4k
L
6.8µH
590k
GND
330pF
ON OFF
LT3481
3481 TA02
RUN/SS BOOST