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S8520 Datasheet(PDF) 24 Page - Seiko Instruments Inc

Part No. S8520
Description  STEP-DOWN, PWM CONTROL or PWM / PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
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Maker  SII [Seiko Instruments Inc]
Homepage  http://www.sii.co.jp
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 24 page
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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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