MIC5219 500mA-Peak Output LDO Regulator - Micrel - #9

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June 2009 9 M0371-061809 Micrel, Inc. MIC5219 Applications Information The MIC5219 is designed for 150mA to 200mA output current applications where a high current spike (500mA) is needed for short, start-up conditions. Basic application of the device will be discussed initially followed by a more detailed discussion of higher current applications. Enable/Shutdown Forcing EN (enable/shutdown) high (>2V) enables the regulator. EN is compatible with CMOS logic. If the enable/ shutdown feature is not required, connect EN to IN (supply input). See Figure 5. Input Capacitor A 1ìF capacitor should be placed from IN to GND if there is more than 10 inches of wire between the input and the AC filter capacitor or if a battery is used as the input. Output Capacitor An output capacitor is required between OUT and GND to prevent oscillation. The minimum size of the output capacitor is dependent upon whether a reference bypass capacitor is used. 1ìF minimum is recommended when CBYP is not used (see Figure 5). 2.2ìF minimum is recommended when CBYP is 470pF (see Figure 6). For applications < 3V, the output capacitor should be increased to 22ìF minimum to reduce start-up overshoot. Larger values improve the regulator’s transient response. The output capacitor value may be increased without limit. The output capacitor should have an ESR (equivalent series resistance) of about 1Ù or less and a resonant frequency above 1MHz. Ultra-low-ESR capacitors could cause oscillation and/or underdamped transient response. Most tantalum or aluminum electrolytic capacitors are adequate; film types will work, but are more expensive. Many aluminum electrolytics have electrolytes that freeze at about –30°C, so solid tantalums are recommended for operation below –25°C. At lower values of output current, less output capacitance is needed for stability. The capacitor can be reduced to 0.47ìF for current below 10mA, or 0.33ìF for currents below 1mA. No-Load Stability The MIC5219 will remain stable and in regulation with no load (other than the internal voltage divider) unlike many other voltage regulators. This is especially important in CMOS RAM keep-alive applications. Reference Bypass Capacitor BYP is connected to the internal voltage reference. A 470pF capacitor (CBYP) connected from BYP to GND quiets this reference, providing a significant reduction in output noise (ultra-low-noise performance). CBYP reduces the regulator phase margin; when using CBYP, output capacitors of 2.2ìF or greater are generally required to maintain stability. The start-up speed of the MIC5219 is inversely proportional to the size of the reference bypass capacitor. Applications requiring a slow ramp-up of output voltage should consider larger values of CBYP. Likewise, if rapid turn-on is necessary, consider omitting CBYP. Thermal Considerations The MIC5219 is designed to provide 200mA of continuous current in two very small profile packages. 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 thermal resistance, junction-to-ambient, of the device and the following basic equation. PD (max) = TJ (max) - TA ( ) èJA TJ(max) is the maximum junction temperature of the die, 125°C, and TA is the ambient operating temperature. èJA is layout dependent; Table 1 shows examples of thermal resistance, junction-to-ambient, for the MIC5219. Package èJA Recommended èJA 1" Square èJC Minimum Footprint 2oz. Copper MM8® (MM) 160°C/W 70°C/W 30°C/W SOT-23-5 (M5) 220°C/W 170°C/W 130°C/W 2×2 MLF® (ML) 90°C/W — — 2×2 Thin MLF® (MT) 90°C/W — — Table 1. MIC5219 Thermal Resistance The actual power dissipation of the regulator circuit can be determined using one simple equation. PD = (VIN – VOUT) IOUT + VIN IGND Substituting PD(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, if we are operating the MIC5219-3.3BM5 at room temperature, with a minimum footprint layout, we can determine the maximum input voltage for a set output current. PD(max) = (125°C - 25°C) 220°C / W PD (max) = 455mW The thermal resistance, junction-to-ambient, for the minimum footprint is 220°C/W, taken from Table 1. The maximum power dissipation number cannot be exceeded for proper operation of the device. Using the output voltage of 3.3V, and an output current of 150mA, we can determine the maximum input voltage. Ground current, maximum of 3mA for 150mA of output current, can be taken from the “Electrical Characteristics” section of the data sheet. 455mW = (VIN – 3.3V) × 150mA + VIN × 3mA 455mW = (150mA) × VIN + 3mA × VIN – 495mW 950mW = 153mA × VIN VIN = 6.2VMAX Therefore, a 3.3V application at 150mA of output current can accept a maximum input voltage of 6.2V in a SOT-23-5 package. For a full discussion of heat sinking and thermal effects on voltage regulators, refer to the “Regulator Thermals” section of Micrel’s Designing with Low-Dropout Voltage Regulators handbook.