Datasheet

LT8641
21
Rev B
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APPLICATIONS INFORMATION
Thermal Considerations and Peak Output Current
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT8641. The ground pins on the bottom of the package
should 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 LT8641.
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 LT8641 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
LT8641 power dissipation by the thermal resistance from
junction to ambient.
The internal overtemperature protection monitors the
junction temperature of the LT8641. If the junction tem-
perature reaches approximately 160°C, the LT8641 will
stop switching and indicate a fault condition until the
temperature drops about 1°C cooler.
Temperature rise of the LT8641 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 5 shows examples of how case temperature
rise can be managed by reducing V
IN
, switching frequency,
or load.
The LT8641’s internal power switches are capable of safely
delivering up to 5A of peak output current. However, due
to thermal limits, the package can only handle 5A loads
for short periods of time. This time is determined by how
quickly the case temperature approaches the maximum
junction rating. Figure 6 shows an example of how case
temperature rise changes with the duty cycle of a 1kHz
pulsed 5A load.
The LT8641’s top switch current limit decreases with
higher duty cycle operation for slope compensation. This
also limits the peak output current the LT8641 can deliver
for a given application. See curve in Typical Performance
Characteristics.
Figure 4. Reverse V
IN
Protection
V
IN
V
IN
D1
LT8641
EN/UV
8641 F04
GND
DC2373A DEMO BOARD
V
IN
= 12V, f
SW
= 1MHz
V
IN
= 24V, f
SW
= 1MHz
V
IN
= 12V, f
SW
= 2MHz
V
IN
= 24V, f
SW
= 2MHz
LOAD CURRENT (A)
0
0.5
1
1.5
2
2.5
3
3.5
0
10
20
30
40
50
60
CASE TEMPERATURE RISE (°C)
Case Temperature Rise
8641 F05
Figure 5. Case Temperature Rise
Figure 6. Case Temperature Rise vs 5A Pulsed Load
DC2373A DEMO BOARD
V
IN
= 12V
V
OUT
= 5V
f
SW
= 2MHz
STANDBY LOAD = 0.25A
1kHz PULSED LOAD = 5A
DUTY CYCLE OF 5A LOAD
0
0.2
0.4
0.6
0.8
0
10
20
30
40
50
60
70
80
90
CASE TEMPERATURE RISE (°C)
Pulsed Load
8641 F06
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