MIC5254 Datasheet, PDF (10/11 Pages) Micrel Semiconductor – DUAL 150MA UCAP LDO WITH ERROR FLAG OUTPUTS

MIC5254
Each output is rated for 150mA of output current, but the
application may limit the amount of output current based on
the total power dissipation and the ambient temperature. A
typical application may call for one 3.3V output and one 2.5V
output from a single Li-Ion battery input. This input can be as
high as 4.2V.
When operating at high ambient temperatures, the output
current may be limited. When operating at an ambient of
60°C, the maximum power dissipation of the package is
calculated as follows:
P_D(max)
=


125°C − 60°C^
200°C/W 
P_D = 325mW
For the application mentioned above, if regulator 1 is sourcing
150mA, it contributes the following to the overall power
dissipation:
P_D(reg2) = (V_IN – V_OUT)I_OUT + V_IN × I_GND
P_D(reg1) = (4.2V – 3.3V)150mA + 4.2V × 100µA
Micrel
P_D(reg1) = 135.5mW
Since the total power dissipation allowable is 325mW, the
maximum power dissipation of the second regulator is limited
to:
P_D(max) = P_D(reg1) + P_D(reg2)
325mW = 135.5mW + P_D(reg2)
P_D(reg2) = 189.5mW
The maximum output current of the second regulator can be
calculated using the same equations but solving for the
output current (ground current is constant over load and
simplifies the equation):
P_D(reg2) = (V_IN – V_OUT)I_OUT + V_IN × I_GND
189.5mW = (4.2V – 2.5V)I_OUT + 4.2V × 100µA
I_OUT = 111.2mA
The second output is limited to 110mA due to the total power
dissipation of the system when operating at 60°C ambient
temperature.
MIC5254
10
September 2003