MIC5312 Datasheet, PDF (10/11 Pages) Micrel Semiconductor – LowQ Mode Dual 300mA LDO with Integrated POR

Micrel, Inc.
MIC5312
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
February 2005
10
M9999-021105
[phone]