Rev.1.10

- 10 -

ILxmax

ILxmin

Ton

Toff

T=1/fosc

Tf

IL

Discontinuous Mode

t

ILxmax

ILxmin

Ton

Toff

T=1/fosc

t

IL

Iconst

Continuous Mode

<Current through L>

Step 1. Lx Tr. turns on, current, IL=i1 flows, energy is charged in L. In proportion to the on time, Ton, of Lx Tr. IL=i1

increases from IL=ILxmin=0 and reaches ILxmax.

Step 2. When the Lx Tr. turns off, L turns on Shottky diode (SD) and flow IL=i2 to maintain IL = ILxmax.

Step 3. IL=i2 decreases gradually, after Tf passes, IL=ILxmin=0 is true, then SD turns off. Note that in the case of

continuous mode, before IL=ILxmin=0 is true, Toff passes and next cycle starts, then Lx Tr. turns on. In this case,

ILxmin>0, therefore IL increases from IL=ILxmin>0.

With the PWM controller, switching time (fosc) in the time unit is fixed, and by controlling Ton, output voltage is

maintained.

There are also two modes, or discontinuous mode and continuous mode for the PWM inverting switching regulator

depending on the continuous characteristic of inductor current.

×t/L.

Therefore, the electric power, P, which is supplied with input side, can be described as in next formula.

P=

Formula 9

0

Formula 10

0

becomes continuous. In the continuous mode, the deviation of the current equals between Ton and Toff, therefore,

Formula 12

formula:

Formula 13

In this moment, the peak current, ILxmax flowing through the inductor and the driver Tr. is described as follows:

×Ton/L

Formula 14

With the formula 12,13, ILxmax is,

ILxmax = (Ton+Toff)/Toff

Formula 15

external components should be selected.

In the formula 14, peak current ILxmax at discontinuous mode can be calculated. Put Iconst=0 in the formula.

The explanation above is based on the ideal calculation, and the loss caused by Lx switch and external components

is not included. The actual maximum output current is between 50% and 80% of the calculation. Especially, when the

0.3V) of the diode should be considered.