Datasheet

LT8640/LT8640-1
23
Rev.C
For more information www.analog.com
DUTY CYCLE OF 7A LOAD
0
CASE TEMPERATURE RISE (°C)
30
40
50
0.6
0.8
8640 F07
20
10
0
0.2 0.4
60
70
80
90
V
IN
= 12V
V
OUT
= 5V
f
SW
= 2MHz
STANDBY LOAD = 0.25A
1KHz PULSED LOAD = 7A
DC2202A DEMO BOARD
LOAD CURRENT (A)
0
CASE TEMPERATURE RISE (°C)
30
40
50
3
5
8640 F06
20
10
0
1 2 4
60
70
80
V
IN
= 12V, f
SW
= 1MHz
V
IN
= 24V, f
SW
= 1MHz
V
IN
= 12V, f
SW
= 2MHz
V
IN
= 24V, f
SW
= 2MHz
DC2202A DEMO BOARD
Figure6. Case Temperature Rise
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 LT8640/LT8640-1. 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 LT8640/LT8640-1. 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
LT8640/LT8640-1 can be estimated by calculating the total
power loss from an efficiency measurement and subtract
-
ing the inductor loss. The die temperature is calculated by
multiplying the LT8640/LT8640-1 power dissipation by the
thermal resistance from junction to ambient.
The internal overtemperature protection monitors the junc
-
tion temperature of the LT8640/LT8640-1. If the junction
temperature reaches approximately 170°C, the LT8640/
LT8640-1 will stop switching and indicate a fault condition
until the temperature drops about 10°C cooler.
APPLICATIONS INFORMATION
Temperature rise of the LT8640/LT8640-1 is worst when
operating at high load, high V
IN
, and high switching fre-
quency. 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 examples of how
case temperature rise can be managed by reducing V
IN
,
switching frequency, or load.
The LT8640/LT8640-1’s internal power switches are
capable of safely delivering up to 7A of peak output cur
-
rent. However, due to thermal limits, the package can
only handle 7A loads for short periods of time. This time
is determined by how quickly the case temperature ap
-
proaches the maximum junction rating. Figure 6 shows
an example of how case temperature rise changes with
the duty cycle of a 1kHz pulsed 7A load.
The LT8640/LT8640-1’s top switch current limit decreases
with higher duty cycle operation for slope compensa
-
tion. This also limits the peak output current the LT8640/
LT8640-1 can deliver for a given application. See curve
in Typical Performance Characteristics.
Figure7. Case Temperature Rise vs 7A Pulsed Load
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