NR263S Datasheet, PDF (18/29 Pages) Sanken electric – Light Load High Efficiency Synchronous Buck Regulator IC

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8. Design Notes
8.1 External Components
All components are required for matching to the condition of use.
8.1.1 Inductor L1
The Inductor is one of the most important components in the Buck regulators. In order to maintain the stabilized
regulator operation, the Inductor should be carefully selected so it must not saturate or overheat excessively at any
conditions. Please select an inductor with care to six items listed below.
 It is for switching regulator use only
Because the coil for the noise filter (For EMI Countermeasure) has large loss and large heat generation, please do
not use.
 Avoidance of sub-harmonic oscillation
Under the peak detection current control, when the control Duty is more than 0.5 in use conditions, the inductor
current may fluctuate at a frequency that is an integer multiple of switching operation frequency. This phenomenon
is the known as sub-harmonic oscillation and this phenomenon theoretically occurs in the peak detection current
control mode. In order to stabilize the operation, although the inductor current compensation is made internally, the
inductance corresponding to the output voltage should be selected as an application. Specifically, for slope
compensation amount is fixed in the IC, it is necessary to moderate the slope of the inductor current. The ripple
portion of Inductor current ΔI_L and the peak current IL_P are calculated from the following equations:
Large Inductance
small Inductance
Δ
(6)
Δ
(7)
Fig. 8-1 Relationship between the inductance and ripple current ΔI_L
According to the equations, if the inductance of the inductor L is small, both ΔIL and ILp is increased. Consequently,
the inductor current becomes very steep if inductance is too small, so that the operation of the converter might
become unstable. It is necessary to take care of an inductance decrease due to magnetic saturation such as in
overload and load shortage.
(Inductance L calculation in case of "D≧0.5")
The duty control is represented by the ratio of the output voltage V_O and the input voltage V_IN. The control duty will
be 0.5 or more in case that the input voltage V_IN is 10V or less. If the inductor current is used as the continuous
current mode (CCM) in this input / output condition, the ΔI_L in equation (9) is recommended the setting of less than
0.2A in order to avoid the sub-harmonic oscillation (Slope relaxation of the inductor current).
(9)
(Inductance L calculation in case of "D＜0.5")
In the case of "D<0.5", the settable range of the ΔI_L becomes "0.2≦ΔI_L≦1A".
6.8μH that is a reference constant in the Typical Application Circuit is roughly the upper limit of the settable range
of the ΔI_L. It is a setting that is able to give the smallest inductance L. If the ΔI_L becomes smaller, the necessary
inductance L will be larger. Please calculate the inductance L using the equation (9) in the range of "ΔI_L=0.2A-1A".
NR263S series-DSE Rev.1.0
SANKEN ELECTRIC CO.,LTD
18
2016.03.09
http://www.sanken-ele.co.jp/en/
© SANKEN ELECTRIC CO.,LTD. 2016