Datasheet LT6300 (Analog Devices) - 10

LT6300
U U W U APPLICATIO S I FOR ATIO
which looks very much like noise, it is easiest to use the When driving a load, a large percentage of the amplifier RMS values of voltages and currents for estimating the quiescent current is diverted to the output stage and driver power dissipation. The voltage and current levels becomes part of the load current. Figure 7 illustrates the shown for this example are for a full-rate ADSL signal total amount of biasing current flowing between the + and driving 20dBm or 100mWRMS of power on to the 100Ω – power supplies through the amplifiers as a function of telephone line and assuming a 0.5dBm insertion loss in load current. As much as 60% of the quiescent no load the transformer. The quiescent current for the LT6300 is operating current is diverted to the load. set to 10mA per amplifier. At full power to the line the driver power dissipation is: The power dissipated in the LT6300 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 amplifiers are configured to place a RMS)|] • 57mARMS differential signal on to the line. The Class AB output stage PD(FULL) = 192mW + 570mW + 570mW = 1.332W in each amplifier will simultaneously dissipate power in The junction temperature of the driver must be kept less the upper power transistor of one amplifier, while sourc- than the thermal shutdown temperature when processing ing current, and the lower power transistor of the other a signal. The junction temperature is determined from the amplifier, while sinking current. The total device power following expression: dissipation is then: TJ = TAMBIENT (°C) + PD(FULL) (W) • θJA (°C/W) PD = PQUIESCENT + PQ(UPPER) + PQ(LOWER) θJA is the thermal resistance from the junction of the PD = (V+ – V–) • IQ + (V+ – VOUTARMS) • LT6300 to the ambient air, which can be minimized by ILOAD + (V – – VOUTBRMS) • ILOAD heat-spreading PCB metal and airflow through the enclo- With no signal being placed on the line and the amplifier sure as required. For the example given, assuming a biased for 10mA per amplifier supply current, the quies- maximum ambient temperature of 50°C and keeping the cent driver power dissipation is: junction temperature of the LT6300 to 150°C maximum, the maximum thermal resistance from junction to ambient PDQ = 24V • 20mA = 480mW required is: This can be reduced in many applications by operating 150 C – 50 C with a lower quiescent current value. θJA MAX = ° ° = . 75 1 C ° / W ( ) . 1 W 332 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) 6300 F07
Figure 7. IQ vs ILOAD
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