S-8520A20MC-AVFT2X Datasheet, PDF (23/44 Pages) Seiko Instruments Inc – SWITCHING REGULATOR CONTROLLER

STEP-DOWN, PWM CONTROL or PWM / PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.9.0_01
S-8520/8521 Series
 Application Circuits
1. External adjustment of output voltage
In the S-8550/8521 Series, by adding external resistors (RA, RB) and a capacitor (CFB), the output voltage can be
adjusted or be set freely in the range of 1.5 to 6.0 V, as shown in Figure 14. Temperature gradient can be given by
inserting a thermistor in series to RA and RB.
OUT
EXT
VIN
S-8520/8521 Series
VOUT
CC
RA
PWM control or PWM /
R1
+
PFM switching control unit
+
D1
−
−
Oscillation Reference voltage
R2
______
ON/OFF
Circuit
source with soft start
VSS
RB
+
−
Figure 14
Caution The above connection diagram and constant will not guarantee successful operation. Perform
through evaluation using the actual application to set the constant.
The S-8520/8521 Series have an internal impedance of R1 and R2 between the VOUT pin and the VSS pin, as
shown in Figure 14.
Therefore, OUT (the output voltage) is determined by the output voltage value (VOUT) of the S-8520/8521 Series,
and the ratio of the parallel resistance value of external resistance (RB) and internal resistances (R1 + R2) of the IC,
to external resistance (RA). The output voltage is expressed by the following equation:
OUT = VOUT + VOUT × RA ÷ (RB //^*1(R1 + R2))
*1. // shows the combined resistance in parallel.
The voltage accuracy of the OUT set by resistances (RA and RB) is not only affected by the IC’s output voltage
accuracy (VOUT ±2.4 %), but also by the absolute precision of external resistances (RA and RB) in use and the
absolute value deviations of internal resistances (R1 and R2) in the IC.
Let us designate the maximum deviations of the absolute value of RA and RB by RA max. and RB max., respectively,
the minimum deviations by RA min. and RB min., respectively, and the maximum and minimum deviations of the
absolute value of R1 and R2 in the IC by (R1 + R2) max. and (R1 + R2) min., respectively. Then, the minimum
deviation value OUT min. and the maximum deviation value OUT max. of the OUT are expressed by the following
equations:
OUT min. = VOUT × 0.976 + VOUT × 0.976 × RA min. ÷ (RB max. // (R1 + R2) max.)
OUT max. = VOUT × 1.024 + VOUT × 1.024 × RA max. ÷ (RB min. // (R1 + R2) min.)
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