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Rev. 1.11

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change of the current at OFF.

When the output current(IOUT) is relatively small, topen<toff as illustrated in the above diagram. In this case, the

energy is charged in the inductor during the time period of ton and is discharged in its entirely during the time period

of toff, therefore ILmin becomes to zero(ILmin=0). When Iout is gradually increased, eventually, topen becomes to

toff (topen = toff), and when IOUT is further increased, ILmin becomes larger than zero(ILmin>0). The former mode

is

referred to as the discontinuous mode and the latter mode is referred to as continuous mode.

In the continuous mode, when Equation 1 is solved for ton and assumed that the solution is tonc,

tonc =T

When ton<tonc, the mode is the discontinuous mode, and when ton = tonc, the mode is the continuous mode.

n OUTPUT CURRENT AND SELECTION OF EXTERNAL COMPONENTS

When LxTr is ON:

×××Equation 3

When LxTr is OFF:

L

×××Equation 4

Ripple Current is as follows;

wherein, peak current that flows through L, LxTr, and SD is as follows;

¼Equation 7

Consider ILmax, condition of input and output and select external components.

HThe above explanation is directed to the calculation in an ideal case in continuous mode.

n External Components

1. Inductor

Select an inductor that peak current does not exceed ILmax. If larger current than allowable current flows,

magnetic saturation occurs and make transform efficiency worse.

When the load current is same, the smaller value of L, the larger the ripple current.

Provided that the allowable current is large in that case and DC current is small, therefore, for large output current,

efficiency is better than using an inductor with a large value of L and vice versa,

2. Diode