SC4910AITSTRT datasheet(10/19 Pages) SEMTECH

Part #	SC4910AITSTRT

Description	High Performance Secondary Side Controller with Synchronous Rectifier

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Manufacturer	SEMTECH [Semtech Corporation]

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10

 2005 Semtech Corp.

www.semtech.com

SC4910A/B

POWER MANAGEMENT

This is the minimum external slope required to avoid sub-

harmonic oscillation at low input line.

With SC4910, the external slope is very easy to

implement. Referring to Figure 2, R12 is the current sense

resistor. R10 and the internal 20K

Ω resistor divide the

internal slope 1.0V - 2.5V down to the required

compensation slope.

)

f

N

L

R

V

Se

(

)

0

.

1

5

.

2

(

20

)

f

N

L

R

V

Se

(

10

R

S

M

S

IN

S

M

S

IN

•

−

•

−

=

where:

LM - Power transformer magnetizing inductance

fS = Switching frequency

In the example,

Ω

=

•

−

•

−

•

−

•

−

=

240

)

3

E

250

100

6

E

450

21

36

085

.

0

(

)

0

.

1

5

.

2

(

20

)

3

E

250

100

6

E

450

21

36

085

.

0

(

10

R

R10

C36

R22

C22

C13

U5

SC4910

18

17

11

16

6

4

10

20

19

3

1

15

2

5

FB

COMP

OUTB

ISHARE

PHASE

CS

OUTA

RT1

RT2

SS

-SENSE

DELAY

SY NC/EN

VREF

R21

R12

R25

Figure 2

Applications Information (Cont.)

Closed-Loop Compensation

The simplified control-to-output transfer function for the

forward converter with current mode control, for small

value of external slope ( Se

≤ Sn, Sn is on-time slope of

sensed current waveform) is given by:

p

z

vgo

vg

s

1

s

1

G

ω

+

ω

+

=

where

closed

loop

current

with

stage

power

of

gain

DC

R

I

V

nN

G

S

O

S

vgo =

closed

loop

current

with

stage

power

of

pole

ant

min

Do

RC

1

P =

ω

stage

power

of

zero

ESR

C

R

1

ESR

Z =

ω

where

R - Load resistance

C - Output capacitance

RESR - Output capacitors ESR

For the given example above, at low line and R = 0.165

Ω,

C = 2 x 680uF = 1360uF, R

ESR = 17mΩ, therefore:

dB

13

71

.

4

21

20

3

.

3

100

6

G

vgo

=

•

=

Hz

6887

s

/

rad

43253

6

E

1360

X

3

E

17

1

Hz

710

s

/

rad

4456

6

E

1360

X

165

.

0

1

Z

P

=

−

=

ω

=

−

=

ω

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SC4910AITSTRT Datasheet(PDF) 10 Page - Semtech Corporation

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