MIC5238 Ultra-Low Quiescent Current, 150mA µCap LDO Regulator - Micrel - #9

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Micrel, Inc. MIC5238 November 2008 9 M9999-111209 Application Information Enable/Shutdown The MIC5238 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” offmode- current state. In this state, current consumed by the regulator goes nearly to zero. Forcing the enable pin high enables the output voltage. Input Bias Capacitor The input capacitor must be rated to sustain voltages that may be used on the input. An input capacitor may be required when the device is not near the source power supply or when supplied by a battery. Small, surface mount, ceramic capacitors can be used for bypassing. Larger values may be required if the source supply has high ripple. Output Capacitor The MIC5238 requires an output capacitor for stability. The design requires 2.2ìF or greater on the output to maintain stability. The design is optimized for use with low-ESR ceramic chip capacitors. High ESR capacitors may cause high frequency oscillation. The maximum recommended ESR is 3Ù. The output capacitor can be increased without limit. Larger valued capacitors help to improve transient response. X7R/X5R dielectric-type ceramic capacitors are recommended because of their temperature performance. X7Rtype capacitors change capacitance by 15% over their operating temperature range and are the most stable type of ceramic capacitors. Z5U and Y5V dielectric capacitors change value by as much as 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 a X7R ceramic capacitor to ensure the same minimum capacitance over the equivalent operating temperature range. No-Load Stability The MIC5238 will remain stable and in regulation with no load unlike many other voltage regulators. This is especially important in CMOS RAM keep-alive applications. Thermal Considerations The MIC5238 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 thermal resistance of the device and the following basic equation: JA J(max) A D(MAX) T - T P è = 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 the junction-to-ambient thermal resistance for the MIC5238. Package èJA Recommended Minimum Footprint SOT-23-5 235°C/W Table 1. SOT-23-5 Thermal Resistance The actual power dissipation of the regulator circuit can be determined using the equation: PD = (VIN – VOUT) IOUT + VINIGND 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, when operating the MIC5238-1.0BM5 at 50°C with a minimum footprint layout, the maximum input voltage for a set output current can be determined as follows. 235 C/W P 125 C - 50 C D(MAX) ° ° ° = PD(MAX) = 319mW The junction-to-ambient (èJA) thermal resistance for the minimum footprint is 235°C/W, from Table 1. It is important that the maximum power dissipation not be exceeded to ensure proper operation. With very high input-to-output voltage differentials, the output current is limited by the total power dissipation. Total power dissipation is calculated using the following equation: PD = (VIN – VOUT) IOUT + VIN x IGND + VBIAS x IBIAS Since the bias supply draws only 18ìA, that contribution can be ignored for this calculation. If we know the maximum load current, we can solve for the maximum input voltage using the maximum power dissipation calculated for a 50°C ambient, 319mV. PD(MAX) = (VIN – VOUT) IOUT + VIN x IGND 319mW = (VIN – 1V) 150mA + VIN x 2.8mA Ground pin current is estimated using the typical characteristics of the device. 469mW = VIN (152.8mA) VIN = 3.07V For higher current outputs only a lower input voltage will work for higher ambient temperatures. Assuming a lower output current of 20mA, the maximum input voltage can be recalculated: 319mW = (VIN – 1V) 20mA + VIN x 0.2mA 339mW = VIN x 20.2mA VIN = 16.8V Maximum input voltage for a 20mA load current at 50°C ambient temperature is 16.8V. Since the device has a 6V rating, it will operate over the whole input range.