Datasheet

LT8608/LT8608B
17
Rev. D
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APPLICATIONS INFORMATION
to ambient. The LT8608 will stop switching and indicate
a fault condition if safe junction temperature is exceeded.
Temperature rise of the LT8608 is worst when operating
at high load, high V
IN
, and high switching frequency. If
the case temperature is too high for a given application,
then either V
IN
, switching frequency or load current can
be decreased to reduce the temperature to an acceptable
level. Figure 6 shows how case temperature rise can be
managed by reducing V
IN
.
acts as a heat sink thermally. To keep thermal resistance
low, extend the ground plane as much as possible, and add
thermal vias under and near the LT8608 to additional ground
planes within the circuit board and on the bottomside.
Figure 7 shows the basic guidelines for a layout example
that can pass CISPR25 radiated emission test with class
5 limits.
Thermal Considerations
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT8608. Figure 7 shows the recommended component
placement with trace, ground plane, and via locations.
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
layers will spread heat dissipated by the LT8608. Placing
additional vias can reduce thermal resistance further. The
maximum load current should be derated as the ambient
temperature approaches the maximum junction rating.
Power dissipation within the LT8608 can be estimated
by calculating the total power loss from an efficiency
measurement and subtracting the inductor loss. The
die temperature is calculated by multiplying the LT8608
power dissipation by the thermal resistance from junction
Figure 6. Case Temperature Rise vs Load Current
V
IN
= 6V
V
IN
= 12V
V
IN
= 36V
I
OUT
(A)
0.00
0.25
0.50
0.75
1.00
1.25
1.50
0
5
10
15
20
25
30
35
40
45
50
CASE TEMP RISE (°C)
(5V out)
8608 F06
f
SW
= 2MHz
L = 2.2µH
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