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ADP3300ART-5 Arkusz danych(PDF) 7 Page - Analog Devices |
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ADP3300ART-5 Arkusz danych(HTML) 7 Page - Analog Devices |
7 / 8 page REV. B ADP3300 –7– Thermal Overload Protection The ADP3300 is protected against damage due to excessive power dissipation by its thermal overload protection circuit, which limits the die temperature to a maximum of 165 °C. Under extreme conditions (i.e., high ambient temperature and high power dissipation), where die temperature starts to rise above 165 °C, the output current is reduced until die tempera- ture has dropped to a safe level. Output current is restored when the die temperature is reduced. Current and thermal limit protections are intended to protect the device against accidental overload conditions. For normal operation, device power dissipation should be externally limited so that junction temperatures will not exceed 125 °C. Calculating Junction Temperature Device power dissipation is calculated as follows: PD = (VIN – VOUT) ILOAD + (VIN) IGND Where ILOAD and IGND are load current and ground current, VIN and VOUT are input and output voltages respectively. Assuming ILOAD = 50 mA, IGND = 0.5 mA, VIN = 8 V and VOUT = 3.3 V, device power dissipation is: PD = (8 – 3.3) 0.05 + 8 × 0.5 mA = 0.239 W ∆T = T J – TA = PD × θ JA = 0.239 × 165 = 39.4 °C With a maximum junction temperature of 125 °C, this yields a maximum ambient temperature of 85 °C. Printed Circuit Board Layout Consideration Surface mount components rely on the conductive traces or pads to transfer heat away from the device. Appropriate PC board layout techniques should be used to remove heat from the immediate vicinity of the package. The following general guidelines will be helpful when designing a board layout: 1. PC board traces with larger cross section areas will remove more heat. For optimum results, use PC boards with thicker copper and wider traces. 2. Increase the surface area exposed to open air so heat can be removed by convection or forced air flow. 3. Do not use solder mask or silkscreen on the heat dissipating traces because it will increase the junction to ambient thermal resistance of the package. Shutdown Mode Applying a high signal to the shutdown pin or tying it to the input pin will turn the output ON. Pulling the shutdown pin down to 0.3 V or below, or tying it to ground, will turn the output OFF. In shutdown mode, quiescent current is reduced to less than 1 µA. Error Flag Dropout Detector The ADP3300 will maintain its output voltage over a wide range of load, input voltage and temperature conditions. If the output is about to lose regulation, for example, by reducing the supply voltage below the combined regulated output and dropout voltages, the ERR pin will be activated. The ERR output is an open collector that will be driven low. Once set, the ERRor flag’s hysteresis will keep the output low until a small margin of operating range is restored either by raising the supply voltage or reducing the load. APPLICATION CIRCUITS Crossover Switch The circuit in Figure 4 shows that two ADP3300s can be used to form a mixed supply voltage system. The output switches between two different levels selected by an external digital input. Output voltages can be any combination of voltages from the Ordering Guide. ADP3300-5.0 OUT IN GND OUTPUT SELECT 5.0V 0V SD C1 1.0 F ADP3300-3.3 OUT IN GND SD C2 0.47 F VOUT = 5V/3.3V VIN = 5.5V TO 12V + + Figure 4. Crossover Switch Higher Output Current If higher current is needed, an appropriate pass transistor can be used, as in Figure 5, to increase the output current to 1 A. VIN = 6V TO 8V VOUT = 5V @ 1A MJE253* C2 10 F C1 47 F R1 50 *AAVID531002 HEAT SINK IS USED IN OUT ERR GND SD ADP3300-5 + Figure 5. High Output Current Linear Regulator |
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