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ADP3333ARMZ-1.8-RL1 Datasheet(PDF) 10 Page - Analog Devices |
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ADP3333ARMZ-1.8-RL1 Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page ADP3333 Rev. B | Page 10 of 12 APPLICATIONS INFORMATION CAPACITOR SELECTION Output Capacitor The stability and transient response of the LDO is a function of the output capacitor. The ADP3333 is stable with a wide range of capacitor values, types, and ESR (anyCAP). A capacitor as low as 1.0 μF is all that is needed for stability. Larger capacitors can be used if high current surges on the output are anticipated. The ADP3333 is stable with extremely low ESR capacitors (ESR ≈ 0), such as multilayer ceramic capacitors (MLCC) or OSCON. Note that the effective capacitance of some capacitor types falls below the minimum rated value over temperature or with dc voltage. Ensure that the capacitor provides at least 1.0 μF of capacitance over temperature and dc bias. Input Bypass Capacitor An input bypass capacitor is not strictly required but is recom- mended in any application involving long input wires or high source impedance. Connecting a 1.0 μF capacitor from the input to ground reduces the circuit’s sensitivity to printed circuit board (PCB) layout and input transients. If a larger output capacitor is necessary, then a larger value input capacitor is also recommended. OUTPUT CURRENT LIMIT The ADP3333 is short-circuit protected by limiting the pass transistor’s base drive current. The maximum output current is limited to about 1 A (see Figure 17). THERMAL OVERLOAD PROTECTION The ADP3333 is protected against damage due to excessive power dissipation by its thermal overload protection circuit. Thermal protection limits the die temperature to a maximum of 165°C. Under extreme conditions (that is, high ambient temperature and power dissipation) where the die temperature starts to rise above 165°C, the output current is reduced until the die temperature drops to a safe level. Current and thermal limit protections are intended to protect the device against accidental overload conditions. For normal operation, the device’s power dissipation should be externally limited so that the junction temperature does not exceed 125°C. CALCULATING JUNCTION TEMPERATURE Device power dissipation is calculated as follows: PD = (VIN − VOUT) IL + (VIN) IGND where IL and IGND are the load current and ground current, and VIN and VOUT are the input and output voltages, respectively. Assuming the worst-case operating conditions are IL = 300 mA, IGND = 2.0 mA, VIN = 4.0 V, and VOUT = 3.0 V, the device power dissipation is PD = (4.0 V − 3.0 V) 300 mA + (4.0 V) 2.0 mA = 308 mW The package used on the ADP3333 has a thermal resistance of 158°C/W for 4-layer boards. The junction temperature rise above ambient is approximately equal to TJA = 0.308 W × 158°C/W = 48.7°C Therefore, to limit the junction temperature to 125°C, the maximum allowable ambient temperature is TA(MAX) = 125°C − 48.7°C = 76.3°C SHUTDOWN MODE Applying a high signal to the shutdown pin, SD, or connecting it to the input pin, IN, turns the output on. Pulling the shutdown pin to 0.3 V or below, or connecting it to ground, turns the output off. In shutdown mode, the quiescent current is reduced to less than 1 μA. PCB LAYOUT CONSIDERATIONS Use the following general guidelines when designing printed circuit boards: Keep the output capacitor as close as possible to the output and ground pins. Keep the input capacitor as close as possible to the input and ground pins. PCB traces with larger cross sectional areas remove more heat from the ADP3333. For optimum heat transfer, use thick copper with wide traces. Connect the NC pins (Pin 4, Pin 5, Pin 6, and Pin 8) to ground for better thermal performance. The thermal resistance can be decreased by approximately 10% by adding a few square centimeters of copper area to the lands connected to the pins of the LDO. Use additional copper layers or planes to reduce the thermal resistance. Again, connecting the other layers to the GND and NC pins of the ADP3333 is best, but not necessary. When connecting the ground pad to other layers, use multiple vias. |
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