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ADP3338AKCZ-5-REEL1 Datasheet(PDF) 10 Page - Analog Devices |
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ADP3338AKCZ-5-REEL1 Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page ADP3338 Rev. B | Page 10 of 16 APPLICATION INFORMATION CAPACITOR SELECTION Output Capacitor The stability and transient response of the LDO is a function of the output capacitor. The ADP3338 is stable with a wide range of capacitor values, types, and ESR (anyCAP). A capacitor as low as 1 µF is the only requirement for stability. A higher ca- pacitance 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 ADP3338 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 temperature 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 sensitivity of the circuit to PC board layout and input transients. If a larger output capacitor is necessary, a larger value input capacitor is recommended. OUTPUT CURRENT LIMIT The ADP3338 is short-circuit protected by limiting the pass transistor’s base drive current. The maximum output current is limited to approximately 2 A (see Figure 16). THERMAL OVERLOAD PROTECTION The ADP3338 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, such as high ambient temperature and power dissipation where the die temperature starts to rise above 160°C, the output current is reduced until the die temperature has dropped to a safe level. Current and thermal limit protections are intended to protect the device against accidental overload conditions. For normal operation, externally limit the power dissipation of the device so the junction temperature does not exceed 150°C. CALCULATING POWER DISSIPATION Device power dissipation is calculated as PD = (VIN – VOUT) × ILOAD + (VIN × IGND) Where ILOAD and IGND are load current and ground current, and VIN and VOUT are the input and output voltages, respectively. Assuming the worst-case operating conditions are ILOAD = 1.0 A, IGND = 10 mA, VIN = 3.3 V, and VOUT = 2.5 V, the device power dissipation is PD = (3.3 V – 2.5 V) × 1000 mA + (3.3 V × 10 mA) = 833 mW So, 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 48 W 833 . 0 C 85 C 125 ° = ° − ° = θJA PRINTED CIRCUIT BOARD LAYOUT CONSIDERATIONS The thermal resistance, θJA, of the 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 is 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 to which the ADP3338 is mounted affects thermal performance. When mounted to the minimal pads of 2 oz. copper, as shown in Figure 22 (a), θJA is 126.6°C/W. Adding a small copper pad under the ADP3338, as shown in Figure 22 (b), reduces the θJA to 102.9°C/W. Increasing the copper pad to one square inch, as shown in Figure 22 (c), reduces the θJA even further to 52.8°C/W. c ab Figure 22. PCB Layouts |
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