LT3502 LINER [Linear Technology], LT3502 Datasheet - Page 19

• Manufacturer Part Number: LT3502
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
If the output capacitor is different than the recommended
capacitor, stability should be checked across all operat-
ing conditions, including load current, input voltage and
temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load.
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 11 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents fl ow in the LT3502/LT3502A’s V
the catch diode (D1) and the input capacitor (C2). 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.
= VIA
BD
R1
V
IN
FB
R2
SHDN
C2
Figure 11
C1
C3
V
OUT
BST
DA
GND
L1
D1
IN
and SW pins,
3502 F11
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 calculat-
ing the total power loss from an effi ciency 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.
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.
20V TO 40V
V
IN
OFF ON
Figure 12. 15V Step-Down Converter
C2
1μF
V
SHDN
IN
LT3502A
GND
BD
LT3502/LT3502A
BOOST
3502 F12
SW
DA
FB
R2
10k
C3
0.1μF
10V
180k
R1
33μH
L1
22pF
19
C1
10μF
V
15V
500mA
OUT
3502fc