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CS8135 Datasheet(PDF) 7 Page  Cherry Semiconductor Corporation 

CS8135 Datasheet(HTML) 7 Page  Cherry Semiconductor Corporation 
7 / 8 page 7 Application Notes: continued ommended value and work towards a less expensive alternative part for each output. Step 1: Place the completed circuit with the tantalum capacitors of the recommended values in an environmen tal chamber at the lowest specified operating temperature and monitor the outputs with an oscilloscope. A decade box connected in series with capacitor C2 will simulate the higher ESR of an aluminum capacitor. Leave the decade box outside the chamber, the small resistance added by the longer leads is negligible. Step 2: With the input voltage at its maximum value, increase the load current slowly from zero to full load on the output under observation and look for oscillations on the output. If no oscillations are observed, the capacitor is large enough to ensure a stable design under steady state conditions. Step 3: Increase the ESR of the capacitor from zero using the decade box and vary the load current until oscillations appear. Record the values of load current and ESR that cause the greatest oscillation. This represents the worst case load conditions for the output at low temperature. Step 4 : Maintain the worst case load conditions set in step 3 and vary the input voltage until the oscillations increase. This point represents the worst case input voltage condi tions. Step 5: If the capacitor is adequate, repeat steps 3 and 4 with the next smaller valued capacitor. A smaller capaci tor will usually cost less and occupy less board space. If the output oscillates within the range of expected operat ing conditions, repeat steps 3 and 4 with the next larger standard capacitor value. Step 6: Test the load transient response by switching in various loads at several frequencies to simulate its real working environment. Vary the ESR to reduce ringing. Step 7: Remove the unit from the environmental chamber and heat the IC with a heat gun. Vary the load current as instructed in step 5 to test for any oscillations. Once the minimum capacitor value with the maximum ESR is found, a safety factor should be added to allow for the tolerance of the capacitor and any variations in regula tor performance. Most good quality aluminum electrolytic capacitors have a tolerance of ±20% so the minimum value found should be increased by at least 50% to allow for this tolerance plus the variation which will occur at low temper atures. The ESR of the capacitor should be less than 50% of the maximum allowable ESR found in step 3 above. Repeat steps 1 through 7 with the capacitor on the other output, C3. The maximum power dissipation for a dual output regu lator (Figure 1) is: PD(max) = {VIN(max)VOUT1(min)}IOUT1(max)+ {VIN(max)VOUT2(min)}IOUT2(max)+VIN(max)IQ (1) Where VIN(max) is the maximum input voltage, VOUT1(min) is the minimum output voltage from VOUT1, VOUT2(min) is the minimum output voltage from VOUT2, IOUT1(max) is the maximum output current for the application, IOUT2(max) is the maximum output current, for the application, and IQ is the quiescent current the regulator consumes at IOUT(max). Once the value of PD(max) is known, the maximum permis sible value of RQJA can be calculated: RQJA = (2) The value of RQJA can then be compared with those in the package section of the data sheet. Those packages with RQJA's less than the calculated value in equation 2 will keep the die temperature below 150¡C. In some cases, none of the packages will be sufficient to dissipate the heat generated by the IC, and an external heatsink will be required. Figure 1: Dual output regulator with key performance parameters labeled. A heat sink effectively increases the surface area of the package to improve the flow of heat away from the IC and into the surrounding air. Each material in the heat flow path between the IC and the outside environment will have a thermal resistance. Like series electrical resistances, these resistances are summed to determine the value of RQJA. RQJA = RQJC + RQCS + RQSA (3) where RQJC = the junctiontocase thermal resistance, RQCS = the casetoheatsink thermal resistance, and RQSA = the heatsinktoambient thermal resistance. RQJC appears in the package section of the data sheet. Like RQJA, it too is a function of package type. RQCS and RQSA are functions of the package type, heatsink and the inter face between them. These values appear in heat sink data sheets of heat sink manufacturers. Heat Sinks VIN Smart Regulator VOUT1 IOUT1 IIN IQ Control Features } VOUT2 IOUT2 150¡C  T A PD Calculating Power Dissipation in a Dual Output Linear Regulator 
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