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MIC5211-3.3BM6 Datasheet(PDF) 8 Page - Micrel Semiconductor |
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MIC5211-3.3BM6 Datasheet(HTML) 8 Page - Micrel Semiconductor |
8 / 12 page MIC5211 Micrel MIC5211 8 November 2000 is operating from a 7.2V-nominal two-cell lithium-ion battery and both regulators are dropping the voltage to 3.0V, then output current will be limited at higher ambient temperatures. For example, at 70 °C ambient the first regulator can supply 3.0V at 50mA output from a 7.2V supply; however, the second regulator will have limitations on output current to avoid thermal shutdown. The dissipation of the first regulator is: P 7.2V 3V 50mA 7.2V 0.85mA D1 =− () +⋅ P 216mW D1 = Since maximum power dissipation for the dual regulator is 250mW at 70 °C, the second regulator can only dissipate up to 34mW without going into thermal shutdown. The amount of current the second regulator can supply is: P 34mW D2(max) = 7.2V 3V I 34mW OUT2(max) − () = 4.2V I 34mW OUT2(max) ⋅= I8mA OUT2(max) = The second regulator can provide up to 8mA output current, suitable for the keep-alive circuitry often required in hand- held applications. Refer to Application Hint 17 for heat sink requirements when higher power dissipation capability is needed. Refer to De- signing with Low Dropout Voltage Regulators for a more thorough discussion of regulator thermal characteristics. Dual-Voltage Considerations For configurations where two different voltages are needed in the system, the MIC5211 has the option of having two independent output voltages from the same input. For ex- ample, a 3.3V rail and a 5.0V rail can be supplied from the MIC5211 for systems that require both voltages. Important considerations must be taken to ensure proper functionality of the part. The input voltage must be high enough for the 5V section to operate correctly, this will ensure the 3.3V section proper operation as well. Both regulators live off of the same input voltage, therefore the amount of output current each regulator supplies may be limited thermally. The maximum power the MIC5211 can dissipate at room temperature is 455mW, as shown in the “Thermal Considerations” section. If we assume 6V input voltage and 50mA of output current for the 3.3V section of the regulator, then the amount of output current the 5V section can provide can be calculated based on the power dissipa- tion. P D = (VGND – VOUT) IOUT + VGND · IGND P D(3.3V) = (6V – 3.3V) 50mA + 6V · 0.85mA P D(3.3V) = 140.1mW P D(max) = 455mW P D(max) – PD(3.3V) = PD(5V) P D(5V) = 455mW – 140.1mW P D(5V) = 314.9mW Based on the power dissipation allowed for the 5V section, the amount of output current it can source is easily calculated. P D(5V) = 314.9mW 314.9mW = (6V – 5V) I MAX – 6V · IGND (I GND typically adds less than 5% to the total power dissipa- tion and in this case can be ignored) 314.9mW = (6V – 5V) I MAX I MAX = 314.9mA I MAX exceeds the maximum current rating of the device. Therefore, for this condition, the MIC5211 can supply 50mA of output current from each section of the regulator. |
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