Datasheet

ADP7182 Data Sheet
Rev. C | Page 24 of 28
reliable operation, device power dissipation must be externally
limited so that the junction temperatures do not exceed 125°C.
THERMAL CONSIDERATIONS
In most applications, the ADP7182 does not dissipate much heat
due to its high efficiency. However, in applications with high
ambient temperature, and high supply voltage to output voltage
differential, the heat dissipated in the package is large enough that
it can cause the junction temperature of the die to exceed the
maximum junction temperature of 125°C.
When the junction temperature exceeds 150°C, the converter
enters thermal shutdown. It recovers only after the junction
temperature has decreased below 135°C to prevent any permanent
damage. Therefore, thermal analysis for the chosen application
is important to guarantee reliable performance over all conditions.
The junction temperature of the die is the sum of the ambient
temperature of the environment and the temperature rise of the
package due to the power dissipation, as shown in Equation 3.
To guarantee reliable operation, the junction temperature of the
ADP7182 must not exceed 125°C. To ensure that the junction
temperature stays below this maximum value, the user must be
aware of the parameters that contribute to junction temperature
changes. These parameters include ambient temperature, power
dissipation in the power device, and thermal resistances between
the junction and ambient air (θ
JA
). The θ
JA
number is dependent
on the package assembly compounds that are used, and the amount
of copper used to solder the package GND pins to the PCB.
Table 7 and Table 8 show typical θ
JA
values of the 8-lead LFCSP
and 5-lead TSOT packages for various PCB copper sizes. Table 9
shows the typical Ψ
JB
values of the 8-lead LFCSP and 5-lead TSOT.
Table 7. Typical θ
JA
Values of the 8-Lead LFCSP
Copper Size (mm
2
) θ
JA
(°C/W)
25
1
175
100
135.6
500
77.3
1000 65.2
6400 51
1
Device soldered to minimum size pin traces.
Table 8. Typical θ
JA
Values of the 5-Lead TSOT
Copper Size (mm
2
) θ
JA
(°C/W)
0
1
170
50 152
100 146
300 134
500 131
1
Device soldered to minimum size pin traces.
Table 9. Typical Ψ
JB
Values
Model Ψ
JB
(°C/W)
8-lead LFCSP 18.2
5-lead TSOT 43
The junction temperature of the ADP7182 can be calculated by
T
J
= T
A
+ (P
D
× θ
JA
) (3)
where:
T
A
is the ambient temperature.
P
D
is the power dissipation in the die, given by
P
D
= [(V
IN
− V
OUT
) × I
LOAD
] + (V
IN
× I
GND
) (4)
where:
V
IN
and V
OUT
are the input and output voltages, respectively.
I
LOAD
is the load current.
I
GND
is the ground current.
Power dissipation due to ground current is quite small and can be
ignored. Therefore, the junction temperature equation simplifies to
T
J
= T
A
+ {[(V
IN
− V
OUT
) × I
LOAD
] × θ
JA
} (5)
As shown in Equation 5, for a given ambient temperature, input-to-
output voltage differential, and continuous load current, there
exists a minimum copper size requirement for the PCB to ensure
that the junction temperature does not rise above 125°C. Figure 87
to Figure 92 show junction temperature calculations for different
ambient temperatures, power dissipation, and areas of PCB copper.
Heat dissipation from the package can be improved by increasing
the amount of copper attached to the pins of the ADP7182.
Adding thermal planes under the package also improves thermal
performance. However, as listed in Table 7 and Table 8, a point
of diminishing returns is reached eventually, beyond which an
increase in the copper area does not yield significant reduction
in the junction-to-ambient thermal resistance.
140
120
100
80
60
40
20
0
0 1.21.00.80.60.40.2
JUNCTION TEMPERATURE, T
J
(°C)
TOTAL POWER DISSIPATION (W)
6400mm
2
1000mm
2
500mm
2
100mm
2
25mm
2
JEDEC
T
J
MAX
10703-087
Figure 87. Junction Temperature vs. Total Power Dissipation for the
8-Lead LFCSP, T
A
= 25°C