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ADP3339AKCZ-3.3-R7 Datasheet(PDF) 10 Page - Analog Devices |
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ADP3339AKCZ-3.3-R7 Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page ADP3339 Data Sheet Rev. C | 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 ADP3339 is stable with a wide range of capacitor values, types, and ESR (anyCAP). A capacitor as low as 1 μF is all that is needed for stability. A higher capacitance may be necessary if high output current surges are anticipated, or if the output capacitor cannot be located near the output and ground pins. The ADP3339 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 over tempera- ture or with dc voltage. Input 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 μF capacitor from the input to ground reduces the circuit’s sensitivity to PC board layout and input transients. If a larger output capacitor is necessary, a larger value input capacitor is also recommended. OUTPUT CURRENT LIMIT The ADP3339 is short-circuit protected by limiting the pass transistor’s base drive current. The maximum output current is limited to about 3 A. See Figure 16. THERMAL OVERLOAD PROTECTION The ADP3339 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 160°C. Under extreme conditions (that is, high ambient temperature and power dissipation) where the die temperature starts to rise above 160°C, the output current is reduced until the die tempera- ture has dropped 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 150°C. CALCULATING POWER DISSIPATION Device power dissipation is calculated as follows: PD = (VIN – VOUT) × ILOAD + (VIN × IGND) where ILOAD and IGND are the load current and ground current, and VIN and VOUT are the input and output voltages, respectively. Assuming worst-case operating conditions are ILOAD = 1.5 A, IGND = 14 mA, VIN = 3.3 V, and VOUT = 2.5 V, the device power dissipation is PD = (3.3 V – 2.5 V) × 1500 mA + (3.3 V × 14 mA) = 1246 mW Therefore, for a junction temperature of 125°C and a maximum ambient temperature of 85°C, the required thermal resistance from junction to ambient is C/W 1 . 32 W 246 . 1 C 85 C 125 ° = ° − ° = JA θ PRINTED CIRCUIT BOARD LAYOUT CONSIDERATIONS The thermal resistance, θJA, of SOT-223 is determined by the sum of the junction-to-case and the case-to-ambient thermal resistances. The junction-to-case thermal resistance, θJC, is determined by the package design and specified at 26.8°C/W. However, the case-to-ambient thermal resistance is determined by the printed circuit board design. As shown in Figure 22, the amount of copper onto which the ADP3339 is mounted affects thermal performance. When mounted onto the minimal pads of 2 oz. copper (see Figure 22a), θJA is 126.6°C/W. Adding a small copper pad under the ADP3339 (see Figure 22b) reduces the θJA to 102.9°C/W. Increasing the copper pad to 1 square inch (see Figure 22c) reduces the θJA even further, to 52.8°C/W. c ab Figure 22. PCB Layouts Use the following general guidelines when designing printed circuit boards: 1. Keep the output capacitor as close to the output and ground pins as possible. 2. Keep the input capacitor as close to the input and ground pins as possible. 3. PC board traces with larger cross sectional areas remove more heat from the ADP3339. For optimum heat transfer, use thick copper and use wide traces. 4. The thermal resistance can be decreased by adding a copper pad under the ADP3339, as shown in Figure 22b. 5. If possible, use the adjacent area to add more copper around the ADP3339. Connecting the copper area to the output of the ADP3339, as shown in Figure 22c, is best, but thermal performance is improved even if it is connected to other pins. 6. Use additional copper layers or planes to reduce the thermal resistance. Again, connecting the other layers to the output of the ADP3339 is best, but is not necessary. When connecting the output pad to other layers, use multiple vias. |
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