AN-6861

APPLICATION NOTE

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.1 • 6/9/09

4

I

Δ

2

RF

EDC

I

K

I

Δ

=

S

PK

D

I

(Design Example)

Determining the ripple factor as 0.57

2

2

3

()

(90 0.53)

2

2 61 65 10 0.57

503

MIN

INP

MAX

M

INP

SW

RF

VD

L

Pf

K

H

μ

⋅

⋅

==

⋅⋅

×

⋅

=

61

1.28

90 0.53

INP

EDC

MIN

INP

MAX

P

I

A

VD

==

=

⋅

⋅

63

90 0.53

1.46

503 10

65 10

MIN

INP

MAX

MSW

VD

I

A

Lf

−

⋅

Δ=

=

=

×⋅

×

DS

1.28 0.73 2.01

2

PK

EDC

I

I

IA

Δ

=+

=

+

=

22

22

3(

)

(

)

23

0.53

3(1.28)

(0.73)

0.98

3

RMS

MAX

DS

EDC

D

I

II

A

Δ

⎡⎤

=+

⎢⎥

⎣⎦

⎡⎤

=+

=

⎣⎦

[STEP-5] Determine the Sensing Resistor Value

The current sensing resistor value should be determined

considering the over-current protection threshold and the

pulse-by-pulse current limit threshold, as shown in Figure 5.

lower than the pulse-by-pulse current limit level for peak

load condition. It should be lower than the OCP threshold

for nominal load conditions to prevent false triggering of

OCP protection during normal operation.

CS

DS

CS

VI

R

=⋅

Figure 5. Determining Current Sensing Resistor

The peak drain current at minimum input voltage and peak

load condition was obtained from Equation (9) in Step-4.

The peak drain current at minimum input voltage and

nominal load condition is given as:

.

()

:

2(

)

MIN

PK

INN

IN

RO

DS N

MIN

INN

RO

MIN

INN

RO

MIN

MSW

INN

RO

PV

V

I

VV

VV

CCM

Lf

V

V

⋅+

=

⋅

⋅

+

⋅+

(13)

.

2

:

PK

INN

DS N

SW

M

P

I

DCM

fL

⋅

=

⋅

(14)

Whether the converter operates in CCM or DCM at

minimum input voltage and nominal load condition is

determined by:

()

21 :

()

21 :

MIN

INN

RO

INN

M

SW

MIN

INN

RO

MIN

INN

RO

INN

M

SW

MIN

INN

RO

VV

P

L f

CCM

VV

VV

P

Lf

DCM

VV

+

⋅>

⋅

+

⋅<

⋅

(15)

The condition for the sensing resistor is given as:

.

0.5

0.89

CS

PK

DS N

CS

PK

DS

R

I

R

I

<

<

(16)

(Design Example)

For minimum input voltage and

nominal load condition, the operation mode is DCM as:

63

()

2

(115 100)

2 23 503 10

65 10

1

115 100

MIN

INN

RO

INN

M

SW

MIN

INN

RO

VV

PL f

VV

−

+

⋅

⋅

+

=

⋅⋅

×

⋅ ×

⋅

<

⋅

The peak drain current at minimum input voltage and

nominal power condition is given as:

DS.N

36

2

223

1.19

65 10 503 10

PK

INN

SW

M

P

I

A

fL

−

⋅

⋅

==

=

⋅×

⋅

×

The conditions for the sensing resistor are given as:

.

.2

0.5

0.5

0.42

1.19

0.89

0.89

0.44

2.01

CS

PK

DS N

CS

PK

DS P

R

I

R

I

<

==

Ω

<

==

Ω

Thus, a 0.39

Ω resistor is selected for the current-sensing

resistor