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S-8521E35MC-BLUT2x Datasheet(PDF) 24 Page - Seiko Instruments Inc |
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S-8521E35MC-BLUT2x Datasheet(HTML) 24 Page - Seiko Instruments Inc |
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24 / 44 page  STEP-DOWN, PWM CONTROL or PWM / PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER S-8520/8521 Series Rev.9.0_01 24 The voltage accuracy of the OUT cannot be made higher than the output voltage accuracy (VOUT ±2.4%) of the IC itself, without adjusting the RA and RB involved. The closer the voltage value of the output OUT and the output voltage value (VOUT) of the IC are brought to one other, the more the output voltage remains immune to deviations in the absolute accuracy of RA and RB and the absolute value of R1 and R2 in the IC. In particular, to suppress the influence of deviations in R1 and R2 in the IC, a major contributor to deviations in the OUT, the RA and RB must be limited to a much smaller value than that of R1 and R2 in the IC. On the other hand, a reactive current flows through RA and RB. This reactive current must be reduced to a negligible value with respect to the load current in the actual use of the IC so that the efficiency characteristics will not be degraded. This requires that the value of RA and RB be made sufficiently large. However, too large a value (more than 1 M Ω) for the RA and RB would make the IC vulnerable to external noise. Check the influence of this value on actual equipment. There is a tradeoff between the voltage accuracy of the OUT and the reactive current. This should be taken into consideration based on the requirements of the intended application. Deviations in the absolute value of the internal resistances (R1 and R2) in the IC vary with the output voltage of the S-8520/8521 Series, and are broadly classified as follows: Table 10 Output voltage Deviations in the absolute value of R1 and R2 in the IC 1.5 V to 2.0 V 5.16 M Ω to 28.9 MΩ 2.1 V to 2.5 V 4.44 M Ω to 27.0 MΩ 2.6 V to 3.3 V 3.60 M Ω to 23.3 MΩ 3.4 V to 4.9 V 2.44 M Ω to 19.5 MΩ 5.0 V to 6.0 V 2.45 M Ω to 15.6 MΩ When a value of R1 + R2 given by the equation indicated below is taken in calculating the voltage value of the output OUT, a median voltage deviation will be obtained for the OUT. R1 + R2 = 2 ÷ (1 ÷ maximum deviation in absolute value of R1 and R2 + 1 ÷ minimum deviation in absolute value of R1 and R2) Moreover, add a capacitor (CC) in parallel to the external resistance (RA) in order to avoid output oscillations and other types of instability (Refer to Figure 14). Make sure that CC is larger than the value given by the following equation: CC [F] ≥ 1 ÷ (2 × π × RA [Ω] × 7.5 kHz) If a large CC value is selected, a longer soft start time than the one set up in the IC will be set. Caution The above connection diagram and constant will not guarantee successful operation. Perform through evaluation using the actual application to set the constant. |
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