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ADP3334ACP Datasheet(PDF) 8 Page - Analog Devices |
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ADP3334ACP Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page REV. B –8– ADP3334 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 150°C. Calculating Junction Temperature Device power dissipation is calculated as follows: PV V I V I DINOUT LOAD IN GND =- () + () (8) where ILOAD and IGND are load current and ground current, VIN and VOUT are input and output voltages, respectively. Assuming ILOAD = 400 mA, IGND = 4 mA, VIN = 5.0 V and VOUT = 2.8 V, device power dissipation is: PmA mA mW D =- () + () = 52 8 400 5 0 4 900 .. (9) As an example, the proprietary package used in the ADP3334 has a thermal resistance of 86.6°C/W, significantly lower than a standard SOIC-8 package. Assuming a 4-layer board, the junction temperature rise above ambient temperature will be approximately equal to: DT= . W C W C A J 0 900 86 6 77 9 ¥ ∞ = ∞ ./ . (10) To limit the maximum junction temperature to 150°C, maxi- mum allowable ambient temperature will be: TC / W C AMAX = ∞ - ∞ = ∞ 150 77 9 72 1 C .. (11) The maximum power dissipation versus ambient temperature for each package is shown in Figure 5. AMBIENT TEMPERATURE – C 3.5 –20 0 20 406080 2.5 1.5 1.0 0.5 0 3.0 2.0 158 C/W MSOP 220 C/W MSOP 122 C/W SOIC 86 C/W SOIC 62 C/W LFCSP 48 C/W LFCSP Figure 5. Power Derating Curve 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. As an example, the patented thermal coastline lead frame design of the ADP3334 uniformly minimizes the value of the dominant portion of the thermal resistance. It ensures that heat is con- ducted away by all pins of the package. This yields a very low 86.6°C/W thermal resistance for the SOIC-8 package, without any special board layout requirements, relying only on the normal traces connected to the leads. This yields a 15% improvement in heat dissipation capability as compared to a standard SOIC-8 package. The thermal resistance can be decreased by an addi- tional 10% by attaching a few square centimeters of copper area to the IN or OUT pins of the ADP3334 package. It is not recommended to use solder mask or silkscreen on the PCB traces adjacent to the ADP3334’s pins since it will increase the junction-to-ambient thermal resistance of the package. 0.50 2x VIAS, 0.250 35µm PLATING 3.36 0.90 1.80 2.36 1.90 1.40 0.30 0.73 Figure 6. 3 mm x 3 mm LFCSP Pad Pattern (Dimensions shown in millimeters) LFCSP Layout Considerations The LFCSP package has an exposed die paddle on the bottom, which efficiently conducts heat to the PCB. In order to achieve the optimum performance from the LFCSP package, special consideration must be given to the layout of the PCB. Use the following layout guidelines for the LFCSP package. 1. The pad pattern is given in Figure 6. The pad dimension should be followed closely for reliable solder joints while maintaining reasonable clearances to prevent solder bridging. 2. The thermal pad of the LFCSP package provides a low ther- mal impedance path (approximately 20°C/W) to the PCB. Therefore the PCB must be properly designed to effectively conduct the heat away from the package. This is achieved by adding thermal vias to the PCB, which provide a thermal path to the inner or bottom layers. See Figure 5 for the rec- ommended via pattern. Note that the via diameter is small to prevent the solder from flowing through the via and leaving voids in the thermal pad solder joint. Note that the thermal pad is attached to the die substrate, so the thermal planes that the vias attach the package to must be electrically isolated or connected to VIN. Do NOT con- nect the thermal pad to ground. |
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