MIC5254_07 Datasheet, PDF (10/11 Pages) Micrel Semiconductor – Dual 150mA μCap LDO with Error Flag Outputs

Micrel, Inc.
125°C and T_A is the ambient operating temperature of
the die. θ_JA is layout dependent. Table 1 shows the
typical thermal resistance for a minimum footprint layout
for the MIC5254.
Package
MSOP-10
θ_JA at Recommended
Minimum Footprint
200°C
Table 1. Thermal Resistance
The actual power dissipation of each regulator output
can be calculated using the following simple equation:
P_D = (V_IN – V_OUT) I_OUT + V_IN × I_GND
Each regulator contributes power dissipation to the
overall power dissipation of the package.
P_D(total) = P_D(reg1) + P_D(reg2)
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
MIC5254
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
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.
December 2007
10
M9999-121107