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S8540 Datasheet(PDF) 21 Page - Seiko Instruments Inc |
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S8540 Datasheet(HTML) 21 Page - Seiko Instruments Inc |
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21 / 36 page  STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER Rev.4.0_01 S-8540/8541 Series 21 RA, RB must be RA + RB ≤ 2 MΩ and the ratio of RA to RB should be set so that the FB pin is 1.0 V. Add a capacitor (CFB) in parallel to RA to prevent unstable operation like output oscillation. Set the CFB so that f = 1/(2 × πCFB × RA) is 0.1 to 20 kHz (normally 10 kHz). e.g. When VOUT = 3.0 V, RA = 200 k Ω, RB = 100 kΩ, then CFB = 100 pF. The precision of output voltage (VOUT) determined by RA, RB is affected by the precision of the voltage at the FB pin (1 V ± 2.0%), the precision of RA and RB, current input to the FB pin, and IC power supply voltage VDD. Suppose that the FB pin input current is 0 nA, and that the maximum absolute values of the external resistors RA and RB are RA max. and RB max, and the minimum absolute values of the external resistors RA and RB are RA min. and RB min., and that the output voltage shift due to the VDD voltage dependency is ΔV, the minimum value VOUT min. and maximum value VOUT max. of the output voltage VOUT variation is calculated by the following formula: VOUTmin. = (1 + max. R min. R B A ) × 0.98 − ΔV [V] VOUTmax. = (1 + Rmax. Rmin. A B ) × 1.02 + ΔV [V] The precision of the output voltage VOUT cannot be made lower than the precision of the IC output voltage without adjustment of external resistors RA and RB. The lower the RA/RB, the less it is affected by the absolute value precision of the external resistors RA and RB. The lower the RA and RB, the less it is affected by the FB pin input current. To suppress the influence of FB pin input current on the variation of output voltage VOUT, the external resistor RB value must be made sufficiently lower than the input impedance of the FB pin, 1 V/50 nA = 20 M Ω max. Waste current flows through external resistors RA and RB. When it is not a negligible value with respect to load current in actual use, the efficiency decreases. The RA and RB values of the external resistors must therefore be made sufficiently high. Evaluation of the influence of the noise is needed in the actual condition If the RA and RB values of resistors are high (1 M Ω or higher) since they are susceptible to external noise. The output voltage VOUT precision and the waste current are in a trade-off relation. They must be considered according to application requests. |
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