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MIC49150-0.9YMM Datasheet(PDF) 11 Page - Micrel Semiconductor |
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MIC49150-0.9YMM Datasheet(HTML) 11 Page - Micrel Semiconductor |
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11 / 13 page  Micrel, Inc. MIC49150 November 2006 11 M9999-111306 ∆T = TJ(max) – TA(max) T J(max) = 125°C TA(max) = maximum ambient operating temp- erature For example, the maximum ambient temperature is 50°C, the ∆T is determined as follows: ∆T = 125°C – 50°C ∆T = 75°C Using Figure 2, the minimum amount of required copper can be determined based on the required power dissipation. Power dissipation in a linear regulator is calculated as follows: PD = VIN × IIN + VBIAS × IBIAS – VOUT × IOUT Using a typical application of 750mA output current, 1.2V output voltage, 1.8V input voltage and 3.3V bias voltage, the power dissipation is as follows: PD = (1.8V) × (730mA) + 3.3V(30mA) – 1.2V(750mA) At full current, a small percentage of the output current is supplied from the bias supply, therefore the input current is less than the output current. PD = 513mW From Figure 2, the minimum current of copper required to operate this application at a ∆T of 75°C is less than 100mm 2. Quick Method Determine the power dissipation requirements for the design along with the maximum ambient temperature at which the device will be operated. Refer to Figure 3, which shows safe operating curves for three different ambient temperatures: 25°C, 50°C and 85°C. From these curves, the minimum amount of copper can be determined by knowing the maxi-mum power dissipation required. If the maximum ambient temperature is 50°C and the power dissipation is as above, 513mW, the curve in Figure 3 shows that the required area of copper is less than 100mm 2. The θJA of this package is ideally 80°C/W, but it will vary depending upon the availability of copper ground plane to which it is attached. Adjustable Regulator Design The MIC49150 adjustable version allows programming the output voltage anywhere between 0.9Vand 5V. Two resistors are used. The resistor value between VOUT and the adjust pin should not exceed 10kΩ. Larger values can cause instability. The resistor values are calculated by: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − × = 1 0.9 V R2 R1 OUT Where VOUT is the desired output voltage. Enable The fixed output voltage versions of the MIC49150 feature an active high enable input (EN) that allows on- off control of the regulator. Current drain reduces to “zero” when the device is shutdown, with only microamperes of leakage current. The EN input has TTL/CMOS compatible thresholds for simple logic interfacing. EN may be directly tied to VIN and pulled up to the maximum supply voltage. |
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