MIC37100/37101/37102
900
COPPER AREA (mm
2
)
Micrel
Using Figure 4, the minimum amount of required copper can
be determined based on the required power dissipation.
Power dissipation in a linear regulator is calculated as fol-
lows:
P
D
= (V
IN
– V
OUT
) I
OUT
+ V
IN
×
I
GND
If we use a 2.5V output device and a 3.3V input at an output
current of 1A, then our power dissipation is as follows:
P
D
= (3.3V – 2.5V)
×
1A + 3.3V
×
11mA
P
D
= 800mW + 36mW
P
D
= 836mW
From Figure 4, the minimum amount of copper required to
operate this application at a
∆T
of 75°C is 160mm
2
.
Quick Method
Determine the power dissipation requirements for the design
along with the maximum ambient temperature at which the
device will be operated. Refer to Figure 5, which shows safe
operating curves for three different ambient temperatures:
25°C, 50°C and 85°C. From these curves, the minimum
amount of copper can be determined by knowing the maxi-
mum power dissipation required. If the maximum ambient
temperature is 50°C and the power dissipation is as above,
836mW, the curve in Figure 5 shows that the required area of
copper is 160mm
2
.
The
θ
JA
of this package is ideally 63°C/W, but it will vary
depending upon the availability of copper ground plane to
which it is attached.
900
800
T = 125°C
J
T
A
= 85°C
50°C 25°C
700
600
500
400
300
200
100
0
0
∆T
JA
=
0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
Figure 4. Copper Area vs. Power SO-8
Power Dissipation
Figure 4 shows copper area versus power dissipation with
each trace corresponding to a different temperature rise
above ambient.
From these curves, the minimum area of copper necessary
for the part to operate safely can be determined. The maxi-
mum allowable temperature rise must be calculated to deter-
mine operation along which curve.
∆T
= T
J
(max) – T
A
(max)
T
J
(max) = 125°C
T
A
(max) = maximum ambient operating temperature
For example, the maximum ambient temperature is 50°C, the
∆T
is determined as follows:
∆T
= 125°C – 50°C
∆T
= 75°C
40°C
50°C
55°C
65°C
75°C
85°C
800
100°C
COPPER AREA (mm
2
)
700
600
500
400
300
200
100
0
0
0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
Figure 5. Copper Area vs. Power-SOIC
Power Dissipation
September 2004
13
M9999-091604
MICREL [ MICREL SEMICONDUCTOR ]
