Micrel, Inc. MIC5312
February 2005
10
M9999-021105
(408) 955-1690
The actual power dissipation of the regulator circuit can
be determined using the equation:
P
DTOTAL
= P
D LDO1
+ P
D LDO2
P
D
LDO1
= (V
IN
-V
OUT1
) x I
OUT1
P
D
LDO2
= (V
IN
-V
OUT2
) x I
OUT2
Substituting P
D(max)
for P
D
and solving for the operating
conditions that are critical to the application will give the
maximum operating conditions for the regulator circuit.
For example, when operating the MIC5312 at 60°C with
a minimum footprint layout, the maximum load currents
can be calculated as follows:
P
D
(max) = (T
J
(max) - T
A
) /θ
JA
P
D
(max) = (125°C - 60°C) / 63°C /W
P
D
(max) = 1.03W
The junction-to-ambient thermal resistance for the
minimum footprint is 63°C/W, from Table 1. The
maximum power dissipation must not be exceeded for
proper operation. Using a lithium-ion battery as the
supply voltage of 4.2V, 1.8V
OUT
/150mA for channel 1
and 2.8V
OUT
/100mA for channel 2, power dissipation
can be calculated as follows:
P
D
LDO1
= (V
IN
-V
OUT1
) x I
OUT1
P
D
LDO1
= (4.2V-1.8V) x 150mA
P
D
LDO1
= 360mW
P
D
LDO2
= (V
IN
-V
OUT2
) x I
OUT2
P
D
LDO1
= (4.2V-2.8V) x 100mA
P
D
LDO1
= 140mW
P
DTOTAL
= P
D LDO1
+ P
D LDO2
P
DTOTAL
= 360mW + 140mW
P
DTOTAL
= 500mW
The calculation shows that we are well below the
maximum allowable power dissipation of 1.03W for a
60° ambient temperature.
After the maximum power dissipation has been
calculated, it is always a good idea to calculate the
maximum ambient temperature for a 125° junction
temperature. Calculating maximum ambient temperature
as follows:
T
A(max)
= T
J(max)
– (P
D
x θ
JA
)
T
A(max)
= 125°C – (500mW x 63°C/W)
T
A(max)
= 93.5°C
For a full discussion of heat sinking and thermal effects
on voltage regulators, refer to the “Regulator Thermals”
section of Micrel’s Designing with Low-Dropout Voltage
Regulators handbook.
This information can be found on Micrel's website at:
http://www.micrel.com/_PDF/other/LDOBk_ds.pdf
