Datasheet LT1739 (Analog Devices) - 10

LT1739
U U W U APPLICATIO S I FOR ATIO
25 compact circuit layout to allow more ports to be imple- mented on any given size PCB. 20 Fortunately xDSL circuit boards use multiple layers of 15 (mA) Q metal for interconnection of components. Areas of metal 10 beneath the LT1739 connected together through several TOTAL I small 13 mil vias can be effective in conducting heat away 5 from the driver package. The use of inner layer metal can free up top and bottom layer PCB area for external compo- 0 –240 –200 –160 –120 –80 –40 0 40 80 120 160 200 240 nent placement. ILOAD (mA) 1739 F07 Figure 8 shows examples of PCB metal being used for heat
Figure 7. IQ vs ILOAD
spreading. These are provided as a reference for what might be expected when using different combinations of At full power to the line the driver power dissipation is: metal area on different layers of a PCB. These examples are PD(FULL) = 24V • 8mA + (12V – 2VRMS) • 57mARMS with a 4-layer board using 1oz copper on each. The most + [|–12V – (– 2VRMS)|] • 57mARMS effective layers for spreading heat are those closest to the LT1739 junction. The small TSSOP and DFN packages are PD(FULL) = 192mW + 570mW + 570mW = 1.332W* very effective for compact line driver designs. Both pack- The junction temperature of the driver must be kept less ages also have an exposed metal heat sinking pad on the than the thermal shutdown temperature when processing bottom side which, when soldered to the PCB top layer a signal. The junction temperature is determined from the metal, directly conducts heat away from the IC junction. following expression: Soldering the thermal pad to the board produces a thermal T resistance from junction to case, θJC, of approximately J = TAMBIENT (°C) + PD(FULL) (W) • θJA (°C/W) 3°C/W. θJA is the thermal resistance from the junction of the LT1739 to the ambient air, which can be minimized by As a minimum, the area directly beneath the package on all heat-spreading PCB metal and airflow through the enclo- PCB layers can be used for heat spreading. Limiting the sure as required. For the example given, assuming a area of metal to just that of the exposed metal heat sinking maximum ambient temperature of 85°C and keeping the pad however is not very effective, particularly if the ampli- junction temperature of the LT1739 to 140°C maximum, fiers are required to dissipate significant power levels. the maximum thermal resistance from junction to ambient This is shown in Figure 8 for both the TSSOP and DFN required is: packages. Expanding the area of metal on various layers significantly reduces the overall thermal resistance. If θ 140 C – 85 C possible, an entire unbroken plane of metal close to the JA MAX = ° ° = . 41 3 C ° / W ( ) . 1 W 332 heat sinking pad is best for multiple drivers on one PCB card. The addition of vias (small 13mil or smaller holes
Heat Sinking Using PCB Metal
which fill during PCB plating) connecting all layers of heat spreading metal also helps to reduce operating tempera- Designing a thermal management system is often a trial tures of the driver. These too are shown in Figure␣ 8. and error process as it is never certain how effective it is until it is manufactured and evaluated. As a general rule,
Important Note: The metal planes used for heat sinking
the more copper area of a PCB used for spreading heat
the LT1739 are electrically connected to the negative
away from the driver package, the more the operating
supply potential of the driver, typically – 12V. These
junction temperature of the driver will be reduced. The
planes must be isolated from any other power planes
limit to this approach however is the need for very
used in the board design.
*Note: Design techniques exist to significantly reduce this value. (See Line Driving Back Termination) 1739fas, sn1739 10