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CS8122 Datasheet(PDF) 7 Page  ON Semiconductor 

CS8122 Datasheet(HTML) 7 Page  ON Semiconductor 
7 / 10 page CS8122 http://onsemi.com 7 APPLICATION NOTES STABILITY CONSIDERATIONS The output or compensation capacitor, COUT, helps determine three main characteristics of a linear regulator: start−up delay, load transient response and loop stability. The capacitor value and type should be based on cost, availability, size and temperature constraints. A tantalum or aluminum electrolytic capacitor is best, since a film or ceramic capacitor with almost zero ESR can cause instability. The aluminum electrolytic capacitor is the least expensive solution, but, if the circuit operates at low temperatures (−25 °C to −40°C), both the value and ESR of the capacitor will vary considerably. The capacitor manufacturers data sheet usually provides this information. The value for the output capacitor COUT shown in Figure 13 should work for most applications, however it is not necessarily the optimized solution. To determine an acceptable value for COUT for a particular application, start with a tantalum capacitor of the recommended value and work towards a less expensive alternative part. Step 1: Place the completed circuit with a tantalum capacitor of the recommended value in an environmental chamber at the lowest specified operating temperature and monitor the outputs with an oscilloscope. A decade box connected in series with the capacitor 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 while observing the output for any oscillations. 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 regulator 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 conditions. Step 5: If the capacitor is adequate, repeat steps 3 and 4 with the next smaller valued capacitor. A smaller capacitor will usually cost less and occupy less board space. If the output oscillates within the range of expected operating 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: Raise the temperature to the highest specified operating temperature. 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 regulator 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 temperatures. The ESR of the capacitor should be less than 50% of the maximum allowable ESR found in step 3 above. CALCULATING POWER DISSIPATION IN A SINGLE OUTPUT LINEAR REGULATOR The maximum power dissipation for a single output regulator (Figure 14) is: PD(max) + VIN(max) * VOUT(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 application, and IQ is the quiescent current the regulator consumes at IOUT(max). Once the value of PD(max) is known, the maximum permissible value of RqJA can be calculated: R qJA + 150 °C * TA PD (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 14. Single Output Regulator With Key Performance Parameters Labeled SMART REGULATOR® Control Features IOUT IIN IQ VIN VOUT 
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