S-8540A15FN-IAAT2Z Datasheet, PDF (21/36 Pages) Seiko Instruments Inc – SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.4.0_01
S-8540/8541 Series
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 kΩ, 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 + ^RAmin. ) × 0.98 − ΔV [V]
RBmax.
VOUTmax. = (1 + ^RAmax. ) × 1.02 + ΔV [V]
RBmin.
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 MΩ 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 MΩ 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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