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MIC5254-SJBMM Datasheet(PDF) 10 Page - Micrel Semiconductor |
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MIC5254-SJBMM Datasheet(HTML) 10 Page - Micrel Semiconductor |
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10 / 11 page  Micrel, Inc. MIC5254 December 2007 10 M9999-121107 125°C and TA is the ambient operating temperature of the die. θJA is layout dependent. Table 1 shows the typical thermal resistance for a minimum footprint layout for the MIC5254. Package θ JA at Recommended Minimum Footprint MSOP-10 200°C Table 1. Thermal Resistance The actual power dissipation of each regulator output can be calculated using the following simple equation: PD = (VIN – VOUT) IOUT + VIN × IGND Each regulator contributes power dissipation to the overall power dissipation of the package. PD(total) = PD(reg1) + PD(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: ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ° ° ° = C/W 200 C 60 - C 125 PD(max) PD = 325mW For the application mentioned above, if regulator 1 is sourcing 150mA, it contributes the following to the overall power dissipation: PD(reg2) = (VIN – VOUT) IOUT + VIN × IGND PD(reg1) = (4.2V – 3.3V)150mA + 4.2V × 100µA PD(reg1) = 135.5mW Since the total power dissipation allowable is 325mW, the maximum power dissipation of the second regulator is limited to: PD(max) = PD(reg1) + PD(reg2)) 325mW = 135.5mW + PD(reg2) PD(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): PD(reg2) = (VIN – VOUT) IOUT + VIN × IGND 189.5mW = (4.2V – 2.5V) IOUT + 4.2V × 100µA IOUT = 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. |
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