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ADP3302 Datasheet(PDF) 7 Page - Analog Devices |
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ADP3302 Datasheet(HTML) 7 Page - Analog Devices |
7 / 9 page ADP3303 –7– REV. Thermal Overload Protection The ADP3303 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 power dissipation), where die temperature starts to rise above 165 °C, the output current is reduced until the die temperature has dropped to a safe level. The 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 = 200 mA, IGND = 2 mA, VIN = 7 V and VOUT = 5.0 V, device power dissipation is: PD = (7 V – 5 V ) 200 mA + (7 V ) 2 mA = 414 mW The proprietary package used in ADP3303 has a thermal resistance of 96 °C/W, significantly lower than a standard 8-lead SOIC package at 170 °C/W. Junction temperature above ambient temperature will be ap- proximately equal to: 0.414 W × 96°C/W = 39.7°C To limit the maximum junction temperature to 125 °C, maxi- mum ambient temperature must be lower than: TAMAX = 125°C – 40°C = 85°C Printed Circuit Board Layout Consideration All surface mount packages rely on the traces of the PC board to conduct heat away from the package. In standard packages, the dominant component of the heat resistance path is the plastic between the die attach pad and the individual leads. In typical thermally enhanced packages, one or more of the leads are fused to the die attach pad, significantly decreasing this component. To make the improvement mean- ingful, however, a significant copper area on the PCB must be attached to these fused pins. The patented thermal coastline lead frame design of the ADP3303 (Figure 22) uniformly minimizes the value of the dominant portion of the thermal resistance. It ensures that heat is conducted away by all pins of the package. This yields a very low, 96 °C/W, thermal resistance for an SO-8 package, without any special board layout requirements, relying on the normal traces connected to the leads. The thermal resistance can be decreased by approximately an additional 10% by attaching a few square cm of copper area to the IN pin of the ADP3303. It is not recommended to use solder mask or silkscreen on the PCB traces adjacent to the ADP3303’s pins since it will increase the junction to ambient thermal resistance of the package. COPPER PADDLE 1 2 3 4 8 7 6 5 COPPER LEAD-FRAME Figure 22. Thermal Coastline Error Flag Dropout Detector The ADP3303 will maintain its output voltage over a wide range of load, input voltage and temperature conditions. If, for example, the output is about to lose regulation by reducing the supply voltage below the combined regulated output and drop- out voltages, the ERR flag will be activated. The ERR output is an open collector, which will be driven low. Once set, the ERR 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. Shutdown Mode Applying a TTL high signal to the shutdown (SD) pin, or tying it to the input pin, will turn the output ON. Pulling SD 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 much less than 1 µA. APPLICATION CIRCUITS Crossover Switch The circuit in Figure 23 shows that two ADP3303s 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. B |
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