
ADD8708
OPERATING TEMPERATURE RANGE
The junction temperature is as follows:
T
J
=
T
AMB
+
θ
JA
×
P
DIS
where:
T
AMB
= ambient temperature.
θ
JA
= junction-to-ambient thermal resistance, in °C/watt.
P
DIS
= power dissipated in the device, in watts.
Rev. 0 | Page 12 of 16
For the ADD8708,
P
DIS
can be calculated by this equation:
P
DIS
=
V
DD
×
I
DQ
+ Σ(
I
OUT X(+)
× (
V
DD
V
OUT X
)) +
Σ(
I
OUT X()
×
V
OUT X
) + (
V
DD
–
V
REG OUT
) ×
I
LOAD
where:
V
DD
×
I
DQ
= nominal system power requirements.
I
OUT X(+)
× (
V
DD
V
OUT X
)= positive-current amplifier load
power dissipation (current comes from
V
DD
).
I
OUT X()
×
V
OUT X
=
negative-current amplifier load power
dissipation (current goes to GND).
(
V
DD
–
V
REG OUT
)
×
I
LOAD
= regulator load power dissipation.
In this example, T
AMB
= 95°C. To calculate P
DIS
, assume the
values in Table 6.
Table 6.
V
OUT
18
V
OUT
17
V
OUT
16
V
OUT
15
V
OUT
14
V
OUT
13
V
OUT
12
V
OUT
11
V
OUT
10
V
OUT
9
V
OUT
8
V
OUT
7
V
OUT
6
V
OUT
5
V
OUT
4
V
OUT
3
V
OUT
2
V
OUT
1
Σ(
I
OUT X(+)
× (
V
DD
V
OUT X
)) + Σ(
I
OUT X(
)
×
V
OUT X
)
V
OUT X
(V)
14.400
11.405
10.627
10.397
10.195
10.080
9.821
9.130
8.611
6.480
6.077
5.098
4.810
4.694
4.435
4.205
3.398
0.202
I
OUT X
(mA)
4.3
5.2
4.4
7.3
7.6
3.9
8.3
7.9
4.5
4.2
5.6
3.3
6.9
5.7
3.5
9.6
9.5
7.2
P (W)
0.00688
0.0239
0.0468
0.0409
0.0441
0.0393
0.0513
0.0543
0.0389
0.0272
0.0556
0.0168
0.0332
0.0644
0.0405
0.113
0.0323
0.00145
0.731
V
DD
× I
DQ
= 16 V × 15 mA = 0.240 W
(
V
DD
–
V
REG OUT
) ×
I
LOAD
= (16 V – 14.4 V) × 5 mA = 0.008 W
P
DIS
= 0.240 W + 0.731 W + 0.008 W =0.979 W
Example 1
Exposed pad soldered down with via θ
JA
= 28.3°C/W:
T
J
= 95°C + (28.3°C/W) × (0.979 W) = 122.7°C
where 150°C is the maximum junction temperature that is
guaranteed before the part breaks down. The maximum process
limit is 125°C. Because
T
J
is < 150°C and < 125°C, this example
demonstrates a condition where the part should perform within
process limits.
Example 2
Exposed pad not soldered down θ
JA
= 47.7°C/W:
T
J
= 95°C + (47.7°C/W) × (0.979 W) = 141.7°C
In this example,
T
J
is < 150°C but > 125°C. Although the part
should not exhibit any damage in this situation, the process
limits have been exceeded. The part may no longer operate
as intended.
These examples show that soldering down the exposed pad is
important for proper heat dissipation. Under the same power-
up and loading conditions, the unsoldered part has a higher
temperature than the soldered part. Therefore, it is strongly
advised that the exposed pad be soldered to V
DD
to maintain
part integrity.