AN-9752

APPLICATION NOTE

© 2011 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.1 • 1/23/13

4

3.2 Adjustable Current Limit

The peak current limit is programmable using a resistor on

the IPK pin. There is an internal current source of 50 µA for

the IPK pin, which generates voltage drop across the

resistor. The voltage of the IPK pin determines the current

limit level. Since the upper and lower clamping voltages of

IPK pin are 3 V and 1.5 V, respectively; the suggested

resistor value for the IPK pin is from 30 kΩ to 60 kΩ.

Figure 7 shows the relationship between the peak current

threshold level is summarized in Table 3. The current limit

following equations:

]

)

3

(

)

5

.

1

[(

5

.

1

1

L

FL

LMT

IPK

H

FL

LMT

IPK

FL

LMT

I

V

I

V

I

−

−

−

−

−

⋅

−

+

⋅

−

=

(5)

]

)

3

(

)

5

.

1

[(

5

.

1

1

L

VA

LMT

IPK

H

VA

LMT

IPK

VA

LMT

I

V

I

V

I

−

−

−

−

−

⋅

−

+

⋅

−

=

(6)

Figure 7.

FSB117H

0.80 A

FSB127H

1.00 A

FSB147H

1.50 A

FSB117H

0.60 A

FSB127H

0.75 A

FSB147H

1.13 A

FSB117H

0.40 A

FSB127H

0.50 A

FSB147H

0.75 A

FSB117H

0.30 A

FSB127H

0.38 A

FSB147H

0.57 A

3.3 Over-Power Protection Level

To determine the IPK pin level, the output over-power

()

η

⋅

⋅













⋅

−

⋅

⋅

=

s

m

on

bulk

on

bulk

LMT

o

F

L

t

V

t

V

I

P

2

2

1

(7)

estimated efficiency for OPP level.

m

on

Bulk

s

on

Bulk

o

LMT

L

t

V

F

t

V

P

I

2

⋅

+

⋅

⋅

⋅

=

η

(8)

With a given transformer specification, the PWM turn-on

time can be determined as (refer to AN-8024):

s

s

Bulk

p

o

p

o

on

F

N

V

N

V

N

V

t

1

⋅

















⋅

+

⋅

⋅

=

(9)

voltage.

μ

μ

4

)

4

(

on

VA

LMT

on

FL

LMT

LMT

t

I

t

I

I

−

⋅

+

⋅

=

−

−

(10)

)

(

)

4

(

)

(

6

D

B

t

C

A

t

I

V

on

on

LMT

IPK

−

⋅

−

+

−

⋅

⋅

=

μ

μ

(11)

Therefore, the resistance of the IPK pin is determined as:

μ

50

IPK

IPK

V

R

=

(12)

Although the upper and lower clamping voltage of the IPK

from 30 kΩ to 60 kΩ.