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CS8221YDFR8 Datasheet(PDF) 4 Page - Cherry Semiconductor Corporation |
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CS8221YDFR8 Datasheet(HTML) 4 Page - Cherry Semiconductor Corporation |
4 / 6 page 4 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 tem- peratures. The ESR of the capacitor should be less than 50% of the maximum allowable ESR found in step 3 above. The maximum power dissipation for a single output regu- lator (Figure 2) is: PD(max) = {VIN(max)ÐVOUT(min)}IOUT(max)+VIN(max)IQ (1) where: VIN(max) is the maximum input voltage, VOUT(min) is the minimum output voltage, IOUT(max) is the maximum output current for the applica- tion, 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) Figure 2. Single output regulator with key performance parameters labeled. 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. 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 junctionÐtoÐcase thermal resistance, RQCS = the caseÐtoÐheatsink thermal resistance, and RQSA = the heatsinkÐtoÐambient 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 CS8221 VOUT IOUT IIN IQ 150¡C - TA PD Calculating Power Dissipation in a Single Output Linear Regulator Application Notes: continued |
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