Datasheet LT6301 (Analog Devices) - 10

LT6301
U U W U APPLICATIO S I FOR ATIO
which looks very much like noise, it is easiest to use the becomes part of the load current. Figure 7 illustrates the RMS values of voltages and currents for estimating the total amount of biasing current flowing between the + and driver power dissipation. The voltage and current levels – power supplies through the amplifiers as a function of shown for this example are for a full-rate ADSL signal load current for one differential driver. As much as 60% of driving 20dBm or 100mWRMS of power on to the 100Ω the quiescent no load operating current is diverted to the telephone line and assuming a 0.5dBm insertion loss in load. the transformer. The quiescent current for the LT6301 is At full power to both lines the total package power dissi- set to 10mA per amplifier. pation is: The power dissipated in the LT6301 is a combination of the P quiescent power and the output stage power when driving D(FULL) = [24V • 8mA + (12V – 2VRMS) • 57mARMS + [|–12V – (– 2V a signal. The two pairs of amplifiers are configured to place RMS)|] • 57mARMS] • 2 a differential signal on two lines. The Class AB output stage PD(FULL) = [192mW + 570mW + 570mW] • 2 in each amplifier will simultaneously dissipate power in = 2.664W* the upper power transistor of one amplifier, while sourc- The junction temperature of the driver must be kept less ing current, and the lower power transistor of the other than the thermal shutdown temperature when processing amplifier, while sinking current. The total device power a signal. The junction temperature is determined from the dissipation is then: following expression: PD = PQUIESCENT + PQ(UPPER) + PQ(LOWER) TJ = TAMBIENT (°C) + PD(FULL) (W) • θJA (°C/W) PD = (V+ – V–) • IQ + (V+ – VOUTARMS) • θ I JA is the thermal resistance from the junction of the LOAD + (V – – VOUTBRMS) • ILOAD LT6301 to the ambient air, which can be minimized by With no signal being placed on the line and the amplifier heat-spreading PCB metal and airflow through the enclo- biased for 10mA per amplifier supply current, the quies- sure as required. For the example given, assuming a cent driver power dissipation is: maximum ambient temperature of 50°C and keeping the P junction temperature of the LT6301 to 150°C maximum, DQ = [24V • 10mA] • 4 = 960mW the maximum thermal resistance from junction to ambient This can be reduced in many applications by operating required is: with a lower quiescent current value or shutting down the part during idle conditions. θ 150 C – 50 C JA MAX = ° ° = . 37 5 C ° / W ( ) When driving a load, a large percentage of the amplifier . 2 W 664 quiescent current is diverted to the output stage and *Design techniques exist to significantly reduce this value (See Line Driving Back Termination). 25 20 15 (mA) Q 10 TOTAL I 5 0 –240 –200 –160 –120 –80 –40 0 40 80 120 160 200 240 ILOAD (mA) (ONE DIFFERENTIAL DRIVER) 6301 F07
Figure 7. IQ vs ILOAD
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