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ADP3338AKC-285 Datasheet(PDF) 7 Page - Analog Devices |
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ADP3338AKC-285 Datasheet(HTML) 7 Page - Analog Devices |
7 / 8 page REV. 0 ADP3338 –7– 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 (i.e., high ambient temperature and power dissipation) where the die temperature starts to rise above 160 °C, the output current will be 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 will not exceed 150 °C. CALCULATING POWER DISSIPATION Device power dissipation is calculated as follows: PV V I V I D IN OUT LOAD IN GND =− ()×+()× Where ILOAD and IGND are load current and ground current, VIN and VOUT are the input and output voltages respectively. Assuming 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: P V V mA V mA mW D = () + () = 3 3 2 5 1000 3 3 10 833 . – .. So, for a junction temperature of 125 °C and a maximum ambi- ent temperature of 85 °C, the required thermal resistance from junction to ambient is: θ JA = °° =° 125 85 0 833 48 CC W CW – . / PRINTED CIRCUIT BOARD LAYOUT CONSIDERATIONS The SOT-223’s thermal resistance, θJA, is determined by the sum of the junction-to-case and the case-to-ambient thermal resistances. The junction-to-case thermal resistance, θ JC, 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 Figures 4a–c, the amount of copper the ADP3338 is mounted to affects the thermal performance. When mounted to 2 oz. copper with just the minimal pads, Figure 4a, the θ JA is 126.6 °C/W. By adding a small copper pad under the ADP3338, Figure 4b, reduces the θJA to 102.9°C/W. Increasing the copper pad to 1 square inch, Figure 4c, reduces the θ JA even further to 52.8 °C/W. a. b. c. Figure 4. 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 will remove more heat from the ADP3338. For optimum heat transfer, specify thick copper and use wide traces. 4. The thermal resistance can be decreased by adding a copper pad under the ADP3338 as shown in Figure 4b. 5. If possible, utilize the adjacent area to add more copper around the ADP3338. Connecting the copper area to the output of the ADP3338, as shown in Figure 4c, is best but will improve thermal performance even if it is connected to other signals. 6. Use additional copper layers or planes to reduce the thermal resistance. Again, connecting the other layers to the output of the ADP3338 is best, but not necessary. When connecting the output pad to other layers use multiple vias. |
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