Datasheet

LT8609/LT8609A/LT8609B
20
Rev. H
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Thermal Considerations and Peak Current Output
For higher ambient temperatures, care should be taken
in the layout of the PCB to ensure good heat sinking of
the LT8609/LT8609A/LT8609B. 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 LT8609/LT8609A/LT8609B. 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 LT8609/LT8609A/LT8609B
can be estimated by calculating the total power loss from
an efficiency measurement and subtracting the induc-
tor loss. The die temperature is calculated by multiplying
the LT8609/LT8609A/LT8609B power dissipation by the
thermal resistance from junction to ambient. The LT8609/
LT8609A/LT8609B will stop switching and indicate a fault
condition if safe junction temperature is exceeded.
Temperature rise of the LT8609/LT8609A/LT8609B 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 fre-
quency or load current can be decreased to reduce the
temperature to an acceptable level. Figure5 shows how
case temperature rise can be managed by reducing V
IN
.
Figure6 shows an example of how case temperature
rise changes with the duty cycle of a 10Hz pulsed 3A
load. Junction temperature will be higher than case
temperature.
LOAD CURRENT (A)
0
0.5
1
1.5
2
2.5
3
0
10
20
30
40
50
60
CASE TEMPERATURE RISE (°C)
8609 F05
V
IN
= 12V
V
IN
= 24V
V
OUT
= 5V
f
SW
= 2MHz
Figure5. Case Temperature Rise vs Load Current
STANDBY LOAD = 50mA
PULSED LOAD = 3A
V
IN
=12V
V
IN
=24V
DUTY CYCLE (%)
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
CASE TEMPERATURE RISE (°C)
8609 F06
V
OUT
= 5V
SW
= 2MHz
f
Figure6. Case Temperature Rise vs 3A Pulsed Load
APPLICATIONS INFORMATION
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