MIC5212 Datasheet, PDF (8/9 Pages) Micrel Semiconductor – Dual 500mA LDO Regulator

MIC5212
Low dropout linear regulators from Micrel are rated to a
maximum junction temperature of 125°C. It is important not
to exceed this maximum junction temperature during opera-
tion of the device. To prevent this maximum junction tempera-
ture from being exceeded, the appropriate ground plane heat
sink must be used.
900
800
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 2. Copper Area vs. Power-SO
Power Dissipation (∆T_JA)
Figure 2 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
Using Figure 2, 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_IN1 – V_OUT1) × I_OUT1 + V_IN1 × I_GND1
+ (V_IN2 – V_OUT2) × I_OUT2 + V_IN2 × I_GND2
With a common 5V input, a 3.3V, 300mA output on LDO 1 and
a 2.5V, 150mA output on LDO 2, power dissipation is as
follows:
P_D = (5V – 3.3V) × 300mA + 5V × 5mA
+ (5V – 2.5V) × 150mA + 5V × 1.8mA
P_D = 0.919W
From Figure 2, the minimum amount of copper required to
operate this application at a ∆T of 75°C is 500mm².
Micrel
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 3, 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,
920mW, the curve in Figure 3 shows that the required area of
copper is 500mm².
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
85°C
50°C 25°C
600
500
400
300
200
100
0
0 0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
Figure 3. Copper Area vs. Power-SO
Power Dissipation (T_A)
MIC5212
8
April 2003