R1282D002A

6

Output Current and Selection of External Components

Output Current of Step-up Circuit and External Components

There are two modes, or discontinuous mode and continuous mode for the PWM step-up switching regulator

depending on the continuous characteristic of inductor current.

× t/L.

dt

t/L

V

P

Ton

0

2

IN

ON

×

=

∫

...................................................................................................Formula 1

With the step-up circuit, electric power is supplied from power source also during off time. In this case, input

dt

)t/L

V

(V

V

P

IN

OUT

Tf

0

IN

OFF

−

×

=

∫

...................................................................................Formula 2

In this formula, Tf means the time of which the energy saved in the inductance is being emitted. Thus average

dt}

)t/L

V

(V

V

dt

t/L

V

{

Toff)

1/(Ton

P

IN

OUT

Tf

0

IN

Ton

0

2

IN

AV

−

×

+

×

×

+

=

∫

∫

............................Formula 3

In PWM control, when Tf

=Toff is true, the inductor current becomes continuos, then the operation of switching

regulator becomes continuous mode.

In the continuous mode, the deviation of the current is equal between on time and off time.

×Ton×Toff/(2×L×(Ton+Toff)), the current flows through the inductor, then the

mode becomes continuous. The continuous current through the inductor is described as

Iconst, then,