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MIC5245-3.3BM5 Datasheet(PDF) 8 Page - Micrel Semiconductor |
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MIC5245-3.3BM5 Datasheet(HTML) 8 Page - Micrel Semiconductor |
8 / 12 page MIC5245 Micrel MIC5245 8 August 2002 Applications Information Enable/Shutdown The MIC5245 comes with an active-high enable pin that allows the regulator to be disabled. Forcing the enable pin low disables the regulator and sends it into a “zero” off-mode- current state. In this state, current consumed by the regulator goes nearly to zero. Forcing the enable pin high enables the output voltage. This part is CMOS and the enable pin cannot be left floating; a floating enable pin may cause an indetermi- nate state on the output. Input Capacitor An input capacitor is not required for stability. A 1 µF input capacitor is recommended when the bulk ac supply capaci- tance is more than 10 inches away from the device, or when the supply is a battery. Output Capacitor The MIC5245 requires an output capacitor for stability. The design requires 1 µF or greater on the output to maintain stability. The capacitor can be a low-ESR ceramic chip capacitor. The MIC5245 has been designed to work specifi- cally with the low-cost, small chip capacitors. Tantalum capacitors can also be used for improved capacitance over temperature. The value of the capacitor can be increased without bound. X7R dielectric ceramic capacitors are recommended be- cause of their temperature performance. X7R-type capaci- tors change capacitance by 15% over their operating tem- perature range and are the most stable type of ceramic capacitors. Z5U and Y5V dielectric capacitors change value by as much 50% and 60% respectively over their operating temperature ranges. To use a ceramic chip capacitor with Y5V dielectric, the value must be much higher than an X7R ceramic or a tantalum capacitor to ensure the same minimum capacitance value over the operating temperature range. Tantalum capacitors have a very stable dielectric (10% over their operating temperature range) and can also be used with this device. Bypass Capacitor A capacitor can be placed from the noise bypass pin to ground to reduce output voltage noise. The capacitor by- passes the internal reference. A 0.01 µF capacitor is recom- mended for applications that require low-noise outputs. Transient Response The MIC5245 implements a unique output stage to dramati- cally improve transient response recovery time. The output is a totem-pole configuration with a P-channel MOSFET pass device and an N-channel MOSFET clamp. The N-channel clamp is a significantly smaller device that prevents the output voltage from overshooting when a heavy load is removed. This feature helps to speed up the transient re- sponse by significantly decreasing transient response recov- ery time during the transition from heavy load (100mA) to light load (100 µA). Active Shutdown The MIC5245 also features an active shutdown clamp, which is an N-channel MOSFET that turns on when the device is disabled. This allows the output capacitor and load to dis- charge, de-energizing the load. Thermal Considerations The MIC5245 is designed to provide 150mA of continuous current in a very small package. Maximum power dissipation can be calculated based on the output current and the voltage drop across the part. To determine the maximum power dissipation of the package, use the junction-to-ambient ther- mal resistance of the device and the following basic equation: P TT D(max) J(max) A JA = − θ T J(max) is the maximum junction temperature of the die, 125 °C, and T A is the ambient operating temperature. θJA is layout dependent; Table 1 shows examples of junction-to- ambient thermal resistance for the MIC5245. Package θθθθθ JA Recommended θθθθθ JA 1" Square θθθθθ JC Minimum Footprint Copper Clad SOT-23-5 (M5) 235 °C/W 185 °C/W 145 °C/W Table 1. SOT-23-5 Thermal Resistance The actual power dissipation of the regulator circuit can be determined using the equation: P D = (VIN – VOUT) IOUT + VIN IGND Substituting P D(max) for PD and solving for the operating conditions that are critical to the application will give the maximum operating conditions for the regulator circuit. For example, when operating the MIC5245-3.3BM5 at 50 °C with a minimum footprint layout, the maximum input voltage for a set output current can be determined as follows: P 125 C 50 C 235 C/W D(max) = °− ° ° P D(max) = 315mW The junction-to-ambient thermal resistance for the minimum footprint is 235 °C/W, from Table 1. The maximum power dissipation must not be exceeded for proper operation. Using the output voltage of 3.3V and an output current of 150mA, the maximum input voltage can be determined. Because this device is CMOS and the ground current is typically 87 µA over the load range, the power dissipation contributed by the ground current is < 1% and can be ignored for this calculation. 315mW = (V IN – 3.3V) 150mA 315mW = V IN × 150mA – 495mW 810mW = V IN × 150mA V IN(max) = 5.4V Therefore, a 3.3V application at 150mA of output current can accept a maximum input voltage of 5.4V in a SOT-23-5 package. For a full discussion of heat sinking and thermal |
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