LT3988
18
3988f
3988 F12
L2L1
D2
D1
C10C9
C2
C8C7
R7R6
R4 R3 R5
U1
C1
C3 C4
components should be as small as possible. Place these
components, along with the inductor and output capacitor,
on the same side of the circuit board and connect them
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. Additionally, keep the SW and BOOST
nodes as small as possible. Figure 12 shows an example
of proper PCB layout.
Thermal Considerations
The die temperature of the LT3988 must be lower than the
maximum rating of 125°C (150°C for the H-grade). 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 LT3988. The maximum load current should be derated
as the ambient temperature approaches 125°C (150°C
applicaTions inForMaTion
Figure 12. Sample PC Board Layout
for the H-grade). The die temperature is calculated by
multiplying the LT3988 power dissipation by the thermal
resistance from junction to ambient. Power dissipation
within the LT3988 can be estimated by calculating the total
power loss from an efficiency measurement and subtract-
ing the catch diode loss. Thermal resistance depends on
the layout of the circuit board, but values from 30°C/W
to 60°C/W are typical.
Related Linear Technology Publications
Application Notes 19, 35, 44, 76 and 88 contain more
detailed descriptions and design information for buck
regulators and other switching regulators. The LT1375
data sheet has a more extensive discussion of output
ripple, loop compensation, and stability testing. Design
Note 318 shows how to generate a dual polarity output
supply using a buck regulator.
