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XR2008ISO8EVB Datasheet(PDF) 11 Page - Exar Corporation |
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XR2008ISO8EVB Datasheet(HTML) 11 Page - Exar Corporation |
11 / 17 page © 2014 Exar Corporation 11 / 17 exar.com/XR1008 Rev 1B XR1008, XR2008 Power Dissipation Power dissipation should not be a factor when operating under the stated 1kΩ load condition. However, applications with low impedance, DC coupled loads should be analyzed to ensure that maximum allowed junction temperature is not exceeded. Guidelines listed below can be used to verify that the particular application will not cause the device to operate beyond it’s intended operating range. Maximum power levels are set by the absolute maximum junction rating of 150°C. To calculate the junction temperature, the package thermal resistance value ThetaJA (θJA) is used along with the total die power dissipation. TJunction = TAmbient + (θJA × PD) Where TAmbient is the temperature of the working environment. In order to determine PD, the power dissipated in the load needs to be subtracted from the total power delivered by the supplies. PD = Psupply - Pload Supply power is calculated by the standard power equation. Psupply = Vsupply × IRMSsupply Vsupply = VS+ - VS- Power delivered to a purely resistive load is: Pload = ((Vload)RMS2)/Rloadeff The effective load resistor (Rloadeff) will need to include the effect of the feedback network. For instance, Rloadeff in Figure 3 would be calculated as: RL || (Rf + Rg) These measurements are basic and are relatively easy to perform with standard lab equipment. For design purposes however, prior knowledge of actual signal levels and load impedance is needed to determine the dissipated power. Here, PD can be found from PD = PQuiescent + PDynamic - Pload Quiescent power can be derived from the speci ed IS values along with known supply voltage, Vsupply. Load power can be calculated as above with the desired signal amplitudes using: (Vload)RMS = Vpeak / √2 ( Iload)RMS = ( Vload)RMS / Rloadeff The dynamic power is focused primarily within the output stage driving the load. This value can be calculated as: PDynamic = (VS+ - Vload)RMS × ( Iload)RMS Assuming the load is referenced in the middle of the power rails or Vsupply/2. The XR1008 is short circuit protected. However, this may not guarantee that the maximum junction temperature (+150°C) is not exceeded under all conditions. Figure 6 shows the maximum safe power dissipation in the package vs. the ambient temperature for the packages available. 0 0.5 1 1.5 -40 -20 0 20 40 60 80 100 120 Ambient Temperature (°C) MSOP-8 SOIC-8 TSOT-5 Figure 6. Maximum Power Derating Driving Capacitive Loads Increased phase delay at the output due to capacitive loading can cause ringing, peaking in the frequency response, and possible unstable behavior. Use a series resistance, RS, between the ampli er and the load to help improve stability and settling performance. Refer to Figure 7. + - Rf Input Output Rg Rs CL RL Figure 7. Addition of RS for Driving Capacitive Loads Table 1 provides the recommended RS for various capacitive loads. The recommended RS values result in approximately <1dB peaking in the frequency response. CL (pF) RS (Ω) -3dB BW (MHz) 10pF 0 22 20pF 100 19 50pF 100 12 100pF 100 10.2 Table 1: Recommended RS vs. CL |
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