NR263S Datasheet, PDF (14/29 Pages) Sanken electric – Light Load High Efficiency Synchronous Buck Regulator IC

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Adjust the capacitance of C_SS so that the excessive overshoot may not occur on the Rising-Waveform of the output
voltage Vo at the start-up. The overshoot occurs when tss is short.If the soft-start is finished before the constant
voltage control follows Vo rising speed,it may become such waveform of Fig7-4. When a capacitance of the C_SS is
increased, though the overshoot will not occur , please understand that the start-up time is longer. In actual operation,
please confirm the Rising-waveform, and adjust the capacitance of the C_SS.
Note: About C_SS discharge to restart
It is explained about discharging of the C_SS capacitor when this IC is restarted such as ON/OFF operation in the EN
terminal. When it was restarted, there is a case where the voltage is remaining in the soft-start capacitor C_SS. In this
IC, it has adopted the forced discharge sequence as shown in the Fig7-5. By the internal impedance, after once
discharging the SS terminal voltage to 0.6V or less, and then resume the soft-start.
Discharge of the capacitor Css, it is discharged by the internal impedance 600 Ω (typ) in the IC.
V_EN(th)=1.1[V](typ)
Discharge impedance
600Ω(typ.)
V_SS2=1.4[V](typ.)
V_SS1=0.6[V](typ.)
Fig. 7-5 Discharge of the capacitor Css at restart
Under the condition that the voltage is remaining in the Css, after the ON-signal is inputted, it takes the time of
“t_discharge+tss” until Vo-waveform rise and stabilize. The soft-start capacitor Css has been charged to the internal
regulator voltage 1.8V.
It considers the discharge from the condition that the soft-start capacitor C_SS has been charged up to 1.8V in the
steady condition. The SS terminal voltage V_SS at optional time t after the start of discharge will be calculated by the
equation (4). For the time t_discharge that the V_SS is discharged to 0.6 V from 1.8 V, it can be calculated by
equation (5).
Ω
(4)
Ω
(5)
When there is a mode for continuous “ON/OFF” operation, consider delay by discharging of the C_SS.
NR263S series-DSE Rev.1.0
SANKEN ELECTRIC CO.,LTD
14
2016.03.09
http://www.sanken-ele.co.jp/en/
© SANKEN ELECTRIC CO.,LTD. 2016