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CS8147YTHA5 Datasheet(PDF) 6 Page - Cherry Semiconductor Corporation |
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CS8147YTHA5 Datasheet(HTML) 6 Page - Cherry Semiconductor Corporation |
6 / 8 page 6 Since both outputs are controlled by the same , the CS8147 is ideal for applications where a sleep mode is required. Using the CS8147, a section of circuitry such as a display and nonessential 5V circuits can be shut down under microprocessor control to conserve energy. The test applications circuit diagram shows an automotive radio application where the display is powered by 10V from VOUT1 and the Tuner IC is powered by 5V from VOUT2. Neither output is required unless both the ignition and the Radio On/OFF switch are on. The secondary output VOUT2 is inherently stable and does not require a compensation capacitor. However a compen- sation capacitor connected between VOUT1 and ground is required for stability in most applications. The output or compensation capacitor 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 instabili- ty. The aluminum electrolytic capacitor is the least expen- sive solution, but, if the circuit operates at low tempera- tures (-25¡C to -40¡C), both the value and ESR of the capac- itor will vary considerably. The capacitor manufacturers data sheet usually provides this information. The value for the output capacitor C2 shown in the test and applications circuit should work for most applications, however it is not necessarily the optimized solution. To determine acceptable value for C2 for a particular application, start with a tantalum capacitor of the recom- mended value and work towards a less expensive alterna- tive part. Step 1: Place the completed circuit with a tantalum capaci- tor of the recommended value in an environmental cham- ber 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 oscil- lations 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 condi- tions. 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 stan- dard 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 oper- ating 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 for each output, 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 capacitors should be less than 50% of the maximum allow- able ESR found in step 3 above. The maximum power dissipation for a dual output regula- tor (Figure 1) is PD(max) = {VIN(max) Ð VOUT1(min)}IOUT1(max) + {VIN(max) Ð 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 appli- cation, IOUT2(max) is the maximum output current, for the appli- cation, and IQ is the quiescent current the regulator consumes at IOUT(max). Once the value of PD(max) is known, the maximum permissi- ble 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. 150¡C - T A PD ENABLE Applications Stability Considerations Calculating Power Dissipation in a Dual Output Linear Regulator |
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