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LT1794CFE Datasheet(PDF) 11 Page - Linear Technology |
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LT1794CFE Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LT1794 Figure 7. IQ vs ILOAD APPLICATIO S I FOR ATIO ILOAD (mA) –240 –200 –160 –120 –80 –40 0 40 80 120 160 200 240 10 15 20 1794 F07 5 0 25 With no signal being placed on the line and the amplifier biased for 10mA per amplifier supply current, the quies- cent driver power dissipation is: PDQ = 24V • 20mA = 480mW This can be reduced in many applications by operating with a lower quiescent current value. When driving a load, a large percentage of the amplifier quiescent current is diverted to the output stage and becomes part of the load current. Figure 7 illustrates the total amount of biasing current flowing between the + and – power supplies through the amplifiers as a function of load current. As much as 60% of the quiescent no load operating current is diverted to the load. At full power to the line the driver power dissipation is: PD(FULL) = 24V • 8mA + (12V – 2VRMS) • 57mARMS + [|–12V – (– 2VRMS)|] • 57mARMS PD(FULL) = 192mW + 570mW + 570mW = 1.332W The junction temperature of the driver must be kept less than the thermal shutdown temperature when processing a signal. The junction temperature is determined from the following expression: TJ = TAMBIENT (°C) + PD(FULL) (W) • θJA (°C/W) θJA is the thermal resistance from the junction of the LT1794 to the ambient air, which can be minimized by heat-spreading PCB metal and airflow through the enclo- sure as required. For the example given, assuming a maximum ambient temperature of 85 °C and keeping the junction temperature of the LT1794 to 140 °C maximum, the maximum thermal resistance from junction to ambient required is: θJA MAX CC W CW () – . ./ = °° =° 140 85 1 332 41 3 Heat Sinking Using PCB Metal Designing a thermal management system is often a trial and error process as it is never certain how effective it is until it is manufactured and evaluated. As a general rule, the more copper area of a PCB used for spreading heat away from the driver package, the more the operating junction temperature of the driver will be reduced. The limit to this approach however is the need for very com- pact circuit layout to allow more ports to be implemented on any given size PCB. Fortunately xDSL circuit boards use multiple layers of metal for interconnection of components. Areas of metal beneath the LT1794 connected together through several small 13 mil vias can be effective in conducting heat away from the driver package. The use of inner layer metal can free up top and bottom layer PCB area for external compo- nent placement. |
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