R1215D

10

Output Current and Selection of External Components

<Basic Circuit>

Inductor

Diode

CL

Lx Tr

i1

i2

GND

<Circuit through L>

Discontinuous Mode

Continuous Mode

ILxmax

ILxmin

ton

toff

t=1/fosc

1/ton

tf

IL

t

ILxmax

ILxmin

ton

toff

t=1/fosc

1/ton

IL

Iconst

t

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.

formula.

dt

L

/

t

V

P

2

IN

ton

0

ON

×

=

∫

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

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

dt

L

/

t

)

V

V

(

V

P

IN

OUT

IN

tf

0

OFF

×

−

×

=

∫

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

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

}

dt

L

/

t

)

V

V

(

V

dt

L

/

t

V

{

)

t

t

/(

1

P

IN

OUT

IN

tf

0

2

IN

ton

0

OFF

ON

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.