MIC37100_07 Datasheet, PDF (13/15 Pages) Micrel Semiconductor – 1A Low-Voltage μCap LDO

Micrel, Inc.
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 maximum allowable temperature rise
must be calculated to determine operation along which
curve.
∆T = T_J(max) – T_A(max)
T_J(max) = 125°C
T_A(max) = maximum ambient operating temp-
erature.
For example, the maximum ambient temperature is
50°C, the ∆T is determined as follows:
∆T = 125°C – 50°C
∆T = 75°C
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 follows:
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 160mm².
MIC37100/37101/37102
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 maximum 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 160mm².
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 ^{TJ = 125°C}
700 T_A = 85°C
600
50°C 25°C
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 2007
13
M9999-090607