LT3508
19
3508fd
High Temperature Considerations
The die temperature of the LT3508 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 LT3508. The maximum load current should be derated
as the ambient temperature approaches 125°C (150°C
for the H-grade). The die temperature is calculated by
multiplying the LT3508 power dissipation by the thermal
resistance from junction to ambient. Power dissipation
within the LT3508 can be estimated by calculating the total
power loss from an effi ciency 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. Die temperature rise was measured
on a 4-layer, 6.5cm × 7.5cm circuit board in still air at a
load current of 1.4A (f
SW
= 700kHz). For a 12V input to
3.3V output the die temperature elevation above ambient
was 13°C; for 24V
IN
to 3.3V
OUT
the rise was 18°C; for
12V
IN
to 5V
OUT
the rise was 14°C and for 24V
IN
to 5V
OUT
the rise was 19°C.
Outputs Greater Than 6V
For outputs greater than 6V, add a resistor of 1k to 2.5k
across the inductor to damp the discontinuous ringing of
the SW node, preventing unintended SW current. The 12V
output circuit in the Typical Applications section shows
the location of this resistor.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for step-down regu-
lators and other switching regulators. The LT1376 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 step-down regulator.
APPLICATIONS INFORMATION
