15
LT1010
1010fc
APPLICATIO S I FOR ATIO
W
U
U
U
Typical thermal calculations for the miniDIP package are
detailed in the following paragraphs.
For 4.8mA supply current (typical at 50
°
C, 30V supply
voltage—see supply current graphs) to the LT1010 at
±
15V, P
D
= power dissipated in the part is equal to:
(30V)(0.0048A) = 0.144W
The rise in junction is then:
(0.144W)(130
°
C/W—This is
θ
JA
for the N package)
= 18.7
°
C.
This means that the junction temperature in 50
°
C ambient
air without driving any current into a load is:
18.7
°
C + 50
°
C = 68.7
°
C
Using the LT1010 to drive 8V DC into a 200
Ω
load using
±
15V power supplies dissipates P
D
in the LT1010 where:
P
V
V
V
R
V
V
V
W
D
OUT
OUT
L
=
(
)
( )
=
(
)( )
Ω
=
+
–
–
.
15
8
8
200
0 280
This causes the LT1010 junction temperature to rise
another (0.280W)(0.130
°
C/W) = 36.4
°
C.
This heats the junction to 68.7
°
C + 36.4
°
C = 105.1
°
C.
Caution: This exceeds the maximum operating tempera-
ture of the device.
An example of 1MHz operation further shows the limita-
tions of the N (or miniDIP) package. For
±
15V operation:
P
D
at I
L
= 0 at 1MHz* = (10mA)(30V) = 0.30W
This power dissipation causes the junction to heat from
50
°
C (ambient in this example) to 50
°
C + (0.3W)
(130
°
C/W) = 89
°
C. Driving 2V
RMS
of 1MHz signal into a
200
Ω
load causes an additional
P
V
W
D
=
Ω
⎛
⎝⎜
⎞
⎠⎟
( )
=
2
200
15 2
0 130
•
–
.
to be dissipated, resulting in another (0.130W)
(0.130
°
C/W) = 16.9
°
C rise in junction temperature to
89
°
C + 16.9
°
C = 105.9
°
C.
Caution: This exceeds the maximum operating tempera-
ture of the device.
Thermal Resistance of DFN Package
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
The following table lists thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with one ounce
copper.
Table 1. DFN Measured Thermal Resistance
COPPER AREA
THERMAL RESISTANCE
TOPSIDE
BACKSIDE
BOARD AREA
(JUNCTION-TO-AMBIENT)
2500 sq mm
2500 sq mm
2500 sq mm
40
°
C/W
1000 sq mm
2500 sq mm
2500 sq mm
45
°
C/W
225 sq mm
2500 sq mm
2500 sq mm
50
°
C/W
100 sq mm
2500 sq mm
2500 sq mm
62
°
C/W
For the DFN package, the thermal resistance junction-to-
case (
θ
JC
), measured at the exposed pad on the back of the
die, is 16
°
C/W.
Continuous operation at the maximum supply voltage and
maximum load current is not practical due to thermal
limitations. Transient operation at the maximum supply is
possible. The approximate thermal time constant for a
2500sq mm 3/32" FR-4 board with maximum topside and
backside area for one ounce copper is 3 seconds. This time
constant will increase as more thermal mass is added (i.e.
vias, larger board, and other components).
For an application with transient high power peaks, aver-
age power dissipation can be used for junction tempera-
ture calculations as long as the pulse period is significantly
less than the thermal time constant of the device and
board.
*See Supply Current vs Frequency graph.