LT3502/LT3502A
19
3502fd
applications inForMation
The loop formed by these components should be as
small as possible and tied to system ground in only one
place. 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, and tie this ground plane to system
ground at one location, ideally at the ground terminal of
the output capacitor C1. The SW and BOOST nodes should
be as small as possible. Finally, keep the FB node small so
that the ground pin and ground traces will shield it from
the SW and BOOST nodes. Include vias near the exposed
GND pad of the LT3502/LT3502A to help remove heat from
the LT3502/LT3502A to the ground plane.
High Temperature Considerations
The die temperature of the LT3502/LT3502A must be lower
than the maximum rating of 125°C. This is generally not
a concern unless the ambient temperature is above 85°C.
For higher temperatures, care should be taken in the layout
of the circuit to ensure good heat sinking of the LT3502/
LT3502A. The maximum load current should be derated
as the ambient temperature approaches 125°C. The die
temperature is calculated by multiplying the LT3502/
LT3502A power dissipation by the thermal resistance from
junction to ambient. Power dissipation within the LT3502/
LT3502A can be estimated by calculating the total power
loss from an efficiency measurement and subtracting
the catch diode loss. Thermal resistance depends on the
layout of the circuit board, but 102°C/W and 110ºC/W are
typical for the (2mm × 2mm) DFN and MS10 packages
respectively.
Figure 12. 15V Step-Down Converter
V
IN
C3
0.1µF
1N4148
OR OTHER
SIMILAR
DIODES
22pF
3502 F12
L1
33µH
10V
SHDN
BOOST
SW
LT3502A
BD
GND
DA
FB
OFF ON
C1
10µF
R2
10k
R1
180k
C2
1µF
V
IN
20V TO 40V
V
OUT
15V
500mA
C4
0.1µF
Outputs Greater Than 7V
Note that for outputs above 7V, the input voltage range
will be limited by the maximum rating of the BOOST pin.
The sum of input and output voltages cannot exceed the
BOOST pin’s 50V rating. The 15V circuit (Figure 12) shows
how to overcome this limitation using an additional Zener
diode.
Other Linear Technology Publications
Application Notes AN19, AN35 and AN44 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.
