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S-8365 Datasheet(PDF) 23 Page - Seiko Instruments Inc |
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S-8365 Datasheet(HTML) 23 Page - Seiko Instruments Inc |
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23 / 63 page  STEP-UP, SUPER-SMALL PACKAGE, 1.2 MHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER Rev.1.1_00 S-8365/8366 Series Seiko Instruments Inc. 23 6. Output voltage setting resistors (RFB1, RFB2), capacitor for phase compensation (CFB) For the S-8365/8366 Series, VOUT can be set to any value by using external divider resistors. Connect the divider resistors between the VOUT and VSS pins. Because VFB = 0.6 V typ., VOUT can be calculated by using the following equation : VOUT = RFB1 + RFB2 RFB2 × 0.6 Connect divider resistors RFB1 and RFB2 as close to the IC as possible to minimize the effects of noise. If noise has an effect, adjust the values of RFB1 and RFB2 so that RFB1 + RFB2 < 100 k Ω. CFB, which is connected in parallel with RFB1, is a capacitor for phase compensation. By setting the zero point (the phase feedback) by adding capacitor CFB to output voltage setting resistor RFB1 in parallel, the phase margin increases, improving the stability of the feedback loop. To effectively use the feedback portion of the phase based on the zero point, define CFB by using the following equation : CFB ≅ L × COUT 3 × RFB1 × VOUT VDD This equation is only a guide. The following explains the optimum setting. To efficiently use the feedback portion of the phase based on the zero point, specify settings so that the phase feeds back at the zero point frequency (fzero) of RFB1 and CFB according to the phase delay at the pole frequency (fpole) of L and COUT. The zero point frequency is generally set slightly higher than the pole frequency. The following equations are used to determine the pole frequency of L and COUT and the zero point frequency set using RFB1 and CFB. fpole ≅ 1 2 × π × L × COUT × VDD VOUT fzero ≅ 1 2 × π × RFB1 × CFB The transient response can be improved by setting the zero point frequency in a lower frequency range. If, however, the zero point frequency is set in a significantly lower range, the gain increases in the range of high frequency and the phase margin decreases. This might result in unstable operation. Determine the proper value after sufficient evaluation with actual application. The typical constants based on our evaluation are shown in Table 15. Table 15 Example of Constant for External Parts VOUT(S) [V] VDD [V] RFB1 [k Ω] RFB2 [k Ω] CFB [pF] L [ µH] COUT [ µF] 1.8 1.2 30 15 100 3.3 10 1.8 1.2 30 15 82 2.2 10 3.32 1.2 68 15 82 3.3 10 3.32 1.2 68 15 68 2.2 10 5.0 1.8 110 15 68 3.3 22 5.0 1.8 110 15 56 2.2 22 9.0 3.3 210 15 39 3.3 22 9.0 3.3 210 15 33 2.2 22 15.0 3.3 360 15 39 3.3 22 15.0 3.3 360 15 33 2.2 22 |
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