Datasheet

12
LT1207
resistance between the layer is small. Copper board stiff-
eners and plated through holes can also be used to spread
the heat generated by the device.
Table 1 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 2oz copper. This data can
be used as a rough guideline in estimating thermal resis-
tance. The thermal resistance for each application will be
affected by thermal interactions with other components as
well as board size and shape.
Table 1. Fused 16-Lead SO Package
TOTAL THERMAL RESISTANCE
TOPSIDE BACKSIDE COPPER AREA (JUNCTION-TO-AMBIENT)
2500 sq. mm 2500 sq. mm 5000 sq. mm 40°C/W
1000 sq. mm 2500 sq. mm 3500 sq. mm 46°C/W
600 sq. mm 2500 sq. mm 3100 sq. mm 48°C/W
180 sq. mm 2500 sq. mm 2680 sq. mm 49°C/W
180 sq. mm 1000 sq. mm 1180 sq. mm 56°C/W
180 sq. mm 600 sq. mm 780 sq. mm 58°C/W
180 sq. mm 300 sq. mm 480 sq. mm 59°C/W
180 sq. mm 100 sq. mm 280 sq. mm 60°C/W
180 sq. mm 0 sq. mm 180 sq. mm 61°C/W
where:
T
J
= Junction Temperature
T
A
= Ambient Temperature
P
D
= Device Dissipation
θ
JA
= Thermal Resistance (Junction-to-Ambient)
As an example, calculate the junction temperature for the
circuit in Figure 8 assuming a 70°C ambient temperature.
The device dissipation can be found by measuring the
supply currents, calculating the total dissipation and then
subtracting the dissipation in the load and feedback
network.
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
The dissipation for each amplifier is:
P
D
= (37.5mA)(30V) – (12V)
2
/(1k||1k) = 0.837W
The total dissipation is P
D
= 1.674W. When a 2500 sq mm
PC board with 2oz copper on top and bottom is used, the
thermal resistance is 40°C/W. The junction temperature T
J
is:
T
J
= (1.674W)(40°C/W) + 70°C = 137°C
The maximum junction temperature for the LT1207 is
150°C, so the heat sinking capability of the board is
adequate for the application.
If the copper area on the PC board is reduced to 280mm
2
the thermal resistance increases to 60°C/W and the junc-
tion temperature becomes:
T
J
= (1.674W)(60°C/W) + 70°C = 170°C
Which is above the maximum junction temperature indi-
cating that the heat sinking capability of the board is
inadequate and should be increased.
COPPER AREA (2oz)
COPPER AREA (mm
2
)
0
THERMAL RESISTANCE (°C/W)
70
60
50
40
30
20
10
0

LT1207 • F07
2000
5000
1000
3000 4000
Figure 7. Thermal Resistance vs Total Copper Area
(Top + Bottom)
Calculating Junction Temperature
The junction temperature can be calculated from the
equation:
T
J
= (P
D
)(θ
JA
) + T
A
–
+
15V
–15V
0.01µF
1k
330Ω
1k 200pF
–12V
12V
f = 2MHz
37.5mA
I
LT1206 • F07
1/2 LT1207
SHDN
Figure 8. Thermal Calculation Example