Datasheet LTM8074 (Analog Devices) - 18

link to page 18 link to page 4 link to page 4 LTM8074
APPLICATIONS INFORMATION
5. Connect all of the GND connections to as large a copper live supply (see Analog Devices Application Note 88 for pour or plane area as possible on the top layer. Avoid a complete discussion). The low loss ceramic capacitor breaking the ground connection between the external combined with stray inductance in series with the power components and the LTM8074. source forms an underdamped tank circuit, and the volt- 6. Use vias to connect the GND copper area to the board’s age at the VIN pin of the LTM8074 can ring to more than internal ground planes. Liberally distribute these GND twice the nominal input voltage, possibly exceeding the vias to provide both a good ground connection and LTM8074’s rating and damaging the part. If the input supply thermal path to the internal planes of the printed circuit is poorly controlled or the LTM8074 is hot-plugged into an board. Pay attention to the location and density of the energized supply, the input network should be designed thermal vias in Figure 5. The LTM8074 can benefit from to prevent this overshoot. This can be accomplished by the heat-sinking afforded by vias that connect to internal installing a small resistor in series to VIN, but the most GND planes at these locations, due to their proximity popular method of controlling input voltage overshoot is to internal power handling components. The optimum add an electrolytic bulk cap to the VIN net. This capacitor’s number of thermal vias depends upon the printed relatively high equivalent series resistance damps the circuit circuit board design. For example, a board might use and eliminates the voltage overshoot. The extra capacitor very small via holes. It should employ more thermal improves low frequency ripple filtering and can slightly vias than a board that uses larger holes. improve the efficiency of the circuit, though it is likely to be the largest component in the circuit.
Thermal Considerations
GND The LTM8074 output current may need to be derated if it GND is required to operate in a high ambient temperature. The CIN amount of current derating is dependent upon the input voltage, output power and ambient temperature. The RUN VIN derating curves given in the Typical Performance Char- RT acteristics section can be used as a guide. These curves were generated by the LTM8074 mounted to a 58cm2 FB 4-layer FR4 printed circuit board. Boards of other sizes and layer count can exhibit different thermal behavior, so VOUT it is incumbent upon the user to verify proper operation GND over the intended system’s line, load and environmental operating conditions. COUT For increased accuracy and fidelity to the actual applica- tion, many designers use FEA (Finite Element Analysis) GND/THERMAL VIAS to predict thermal performance. To that end, Page 2 of 8074 F05 the data sheet typically gives four thermal coefficients:
Figure 5. Layout Showing Suggested External Components, GND Plane and Thermal Vias
θJA – Thermal resistance from junction to ambient θ
Hot-Plugging Safely
JCbottom – Thermal resistance from junction to the bottom of the product case The small size, robustness and low impedance of ceramic θ capacitors make them an attractive option for the input JCtop – Thermal resistance from junction to top of the product case bypass capacitor of LTM8074. However, these capacitors can cause problems if the LTM8074 is plugged into a θJB – Thermal resistance from junction to the printed circuit board. Rev. A 18 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Photo Package Description Typical Application Related Parts