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SS6639

I

PK

I

IN

V

EXT

I

OUT

T

DIS

Charge Co.

Discharge Co.

t

Discontinuous Conduction Mode

I

PK

I

IN

V

EXT

I

OUT

t

Continuous Conduction Mode

At

the

boundary

between

continuous

and

discontinuous modes, the output current (IOB) is

determined by

(

1

*

T

*

L

V

*

2

1

*

V

V

V

I

ON

IN

D

OUT

IN

OB

−













+

=

L

T

)

R

(R

X

ON

S

ON

∗

+

=

resistance

In the discontinuous mode, the switching frequency

(Fsw) is

)

x

(1

*

T

*

V

)

(I

*

)

V

V

(V

*

2(L)

F

2

ON

2

IN

OUT

IN

D

OUT

SW

+

−

+

=

In the continuous mode, the switching frequency is

(

)













−

+

−

+

≅

−

+

−

+

−

+

−

+

=

SW

D

OUT

IN

D

OUT

ON

SW

D

OUT

SW

IN

SW

D

OUT

IN

D

OUT

ON

V

V

V

V

V

V

*

T

1

)]

V

V

V

V

V

(

2

x

[1

*

)

V

V

(V

V

V

V

*

T

1

fsw

where Vsw = switch drop and is proportional to

output current.

INDUCTOR SELECTION

To operate as an efficient energy transfer element,

the inductor must fulfill three requirements. First, the

inductance must be low enough for the inductor to

store adequate energy under the worst case

condition of minimum input voltage and switch ON

time. Second, the inductance must also be high

enough so the maximum current rating of the

SS6639 and the inductor are not exceeded at the

other worst case condition of maximum input voltage

and ON time. Lastly, the inductor must have

sufficiently low DC resistance so excessive power is

not lost as heat in the windings. Unfortunately this is

inversely related to physical size.

Rev.2.01 6/26/2003