R1211x

10

Output Current and Selection of External Components

<Basic Circuit>

i1

CL

i2

Inductor

GND

Diode

<Circuit through L>

Discontinuous Mode

Continuous Mode

IL

ILxmax

ILxmin

Tf

t

Ton

T=1/fosc

Toff

IL

ILxmax

ILxmin

t

Ton

T=1/fosc

Iconst

Toff

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.

dt

t/L

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

t/L

)

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}

t/L

)

V

(V

V

dt

t/L

V

{

)

T

1/(T

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.

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

Toff/L

)

V

(V

/L

V

IN

OUT

ON

IN

×

−

=

×T

................................................................................................... Formula 4

{}

)

V

L

/(2

T

V

)

V

(V

L

2

/

T

V

f

I

OUT

ON

2

IN

IN

OUT

2

ON

2

IN

OSC

OUT

×

×

×

=

−

×

×

×

×

=

.................................................... Formula 5