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SPX2815AR-2.5 Datasheet(PDF) 4 Page - Sipex Corporation |
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SPX2815AR-2.5 Datasheet(HTML) 4 Page - Sipex Corporation |
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4 / 10 page  4 Date: 5/17/04 SPX2815 1.5A Low Dropout Voltage Regulator © Copyright 2004 Sipex Corporation -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 100 1000 10000 100000 1000000 Frequency, Hz Figure 5. Ripple Rejection; Vin=3.3V, Vout=1.8V (adj.), Iload=200mA Figure 6. Current Source LOAD ADJ SPX2815 V IN C 1 OUT IN 4.7µF C 2 I OUT R 1 V REF R 1 I OUT = + V OUT Ripple Rejection Ripple rejection can be improved by adding a capacitor between the ADJ pin and ground as shown in Figure 8. When ADJ pin bypassing is used, the value of the output capacitor required increases to its maximum. If the ADJ pin is not bypassed, the value of the output capacitor can be lowered to 22 µF for an electrolytic aluminum capacitor or 10 µF for a solid tantalum capacitor (Fig 7). However the value of the ADJ-bypass capaci- tor should be chosen with respect to the following equation: C = 1 / ( 6.28 * F R * R1 ) Where C = value of the capacitor in Farads (select an equal or larger standard value), F R = ripple frequency in Hz, R 1 = value of resistor R1 in Ohms. If an ADJ-bypass capacitor is used, the amplitude of the output ripple will be indepen- dent of the output voltage. If an ADJ-bypass capacitor is not used, the output ripple will be proportional to the ratio of the output voltage to the reference voltage: M = V OUT / VREF Where M = multiplier for the ripple seen when the ADJ pin is optimally bypassed. V REF =1.25V Ripple rejection for the adjustable version is shown in Figure 5. Output Voltage The output of the adjustable regulator can be set to any voltage between 1.25V and 15V. The value of V OUT can be quickly approxi- mated using the formula V OUT=1.25 *(R1 + R2)/R1 A small correction to this formula is required depending on the values of resistors R 1 and R2, since the adjustable pin current (approx 50 µA) flows through R 2. When IADJ is taken into account, the formula becomes V OUT = VREF(1+ (R2/R1)) + IADJ * R2 where V REF=1.25V Layout Considerations Parasitic line resistance can degrade load regulation. In order to avoid this, connect R 1 directly to V OUT as illustrated in Figure 13. For the same reason, R 2 should be connected to the negative side of the load. APPLICATION INFORMATION: Continued |
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