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SC4525D

10

Applications Information (Cont.)

the regulator, the current-limit comparator ILIM (Figure

2) will eventually limit the switch current on a cycle-by-

cycle basis. The over-current signal OC goes high, setting

the latch B

(I

current limit and the PWM comparator instead turns off

the switch, then latch B

the soft-start capacitor. If over-current condition persists

or OC becomes asserted more often than PWM over

a period of time, then the soft-start capacitor will be

discharged below 1.9V. At this juncture, comparator B

4

sets the overload latch B

continuously discharged with (I

immediately pulled to ground. The switching regulator is

shut off until the soft-start capacitor is discharged below

1.0V. At this moment, the overload latch is reset. The

soft-start capacitor is recharged and the converter again

undergoes soft-start. The regulator will go through soft-

start, overload shutdown and restart until it is no longer

overloaded.

If the FB voltage falls below 0.8V because of output

overload, then the switching frequency will be reduced.

Frequency foldback helps to limit the inductor current

when the output is hard shorted to ground.

During normal operation, the soft-start capacitor is

charged to 2.4V.

Setting the Output Voltage

The regulator output voltage, V

resistive divider (Figure 1) with its center tap tied to the FB

pin. For a given R

(1)

Minimum On Time Consideration

The operating duty cycle of a non-synchronous step-

down switching regulator in continuous-conduction

mode (CCM) is given by

(2)

CESAT

D

IN

D

O

V

V

V

V

V

D

−

+

+

=













−

=

1

V

0

.

1

V

R

R

O

6

4

1

SW

D

O

L

L

F

)

D

1

(

)

V

V

(

I

⋅

−

⋅

+

=

D

SW

O

D

O

1

F

I

%

20

)

D

1

(

)

V

V

(

L

⋅

⋅

−

⋅

+

=

)

D

1

(

D

I

I

O

CIN

_

RMS

−

⋅

⋅

=













⋅

⋅

+

⋅

D

=

D

O

SW

L

O

C

F

8

1

ESR

I

V

SW

IN

O

IN

F

V

4

I

C

⋅

D

⋅

>

,

R

G

R

G

S

CA

PWM

⋅

≈

)

/

s

Q

/

s

1

()

/

s

1

(

)

C

R

s

1

(

G

V

V

2

n

2

n

p

O

ESR

PWM

c

o

ω

+

ω

+

ω

+

+

=

7

1

Z

5

R

F

2

1

C

π

=

7

1

P

8

R

F

2

1

C

π

=

,

C

R

1

O

ω

,

C

R

1

O

ESR

ω

k

3

.

22

10

28

.

0

10

R

3

7

20

9

.

15

=

⋅

=

−

nF

45

.

0

10

1

.

22

10

16

2

1

C

3

3

5

=

⋅

⋅

⋅

⋅

π

=

pF

12

10

1

.

22

10

600

2

1

C

3

3

8

=

⋅

⋅

⋅

⋅

π

=













⋅

π

⋅

⋅

−

=

O

FB

O

C

S

CA

C

V

V

C

F

2

1

R

G

1

log

20

A

dB

9

.

15

3

.

3

0

.

1

10

22

10

80

2

1

10

1

.

6

28

1

log

20

A

6

3

3

C

=













⋅

⋅

⋅

⋅

⋅

π

⋅

⋅

⋅

⋅

−

=

−

−

m

7

g

10

R

20

C

A

=

CESAT

D

IN

D

O

V

V

V

V

V

D

−

+

+

=













−

=

1

V

0

.

1

V

R

R

O

6

4

1

SW

D

O

L

L

F

)

D

1

(

)

V

V

(

I

⋅

−

⋅

+

=

D

SW

O

D

O

1

F

I

%

20

)

D

1

(

)

V

V

(

L

⋅

⋅

−

⋅

+

=

)

D

1

(

D

I

I

O

CIN

_

RMS

−

⋅

⋅

=













⋅

⋅

+

⋅

D

=

D

O

SW

L

O

C

F

8

1

ESR

I

V

SW

IN

O

IN

F

V

4

I

C

⋅

D

⋅

>

,

R

G

R

G

S

CA

PWM

⋅

≈

)

/

s

Q

/

s

1

()

/

s

1

(

)

C

R

s

1

(

G

V

V

2

n

2

n

p

O

ESR

PWM

c

o

ω

+

ω

+

ω

+

+

=

7

1

Z

5

R

F

2

1

C

π

=

7

1

P

8

R

F

2

1

C

π

=

,

C

R

1

O

ω

,

C

R

1

O

ESR

ω

k

3

.

22

10

28

.

0

10

R

3

7

20

9

.

15

=

⋅

=

−

nF

45

.

0

10

1

.

22

10

16

2

1

C

3

3

5

=

⋅

⋅

⋅

⋅

π

=

pF

12

10

1

.

22

10

600

2

1

C

3

3

8

=

⋅

⋅

⋅

⋅

π

=













⋅

π

⋅

⋅

−

=

O

FB

O

C

S

CA

C

V

V

C

F

2

1

R

G

1

log

20

A

dB

9

.

15

3

.

3

0

.

1

10

22

10

80

2

1

10

1

.

6

28

1

log

20

A

6

3

3

C

=













⋅

⋅

⋅

⋅

⋅

π

⋅

⋅

⋅

⋅

−

=

−

−

m

7

g

10

R

20

C

A

=

CESAT

D

IN

D

O

V

V

V

V

V

D

−

+

+

=













−

=

1

V

0

.

1

V

R

R

O

6

4

1

SW

D

O

L

L

F

)

D

1

(

)

V

V

(

I

⋅

−

⋅

+

=

D

SW

O

D

O

1

F

I

%

20

)

D

1

(

)

V

V

(

L

⋅

⋅

−

⋅

+

=

)

D

1

(

D

I

I

O

CIN

_

RMS

−

⋅

⋅

=













⋅

⋅

+

⋅

D

=

D

O

SW

L

O

C

F

8

1

ESR

I

V

SW

IN

O

IN

F

V

4

I

C

⋅

D

⋅

>

,

R

G

R

G

S

CA

PWM

⋅

≈

)

/

s

Q

/

s

1

()

/

s

1

(

)

C

R

s

1

(

G

V

V

2

n

2

n

p

O

ESR

PWM

c

o

ω

+

ω

+

ω

+

+

=

7

1

Z

5

R

F

2

1

C

π

=

7

1

P

8

R

F

2

1

C

π

=

,

C

R

1

O

ω

,

C

R

1

O

ESR

ω

k

3

.

22

10

28

.

0

10

R

3

7

20

9

.

15

=

⋅

=

−

nF

45

.

0

10

1

.

22

10

16

2

1

C

3

3

5

=

⋅

⋅

⋅

⋅

π

=

pF

12

10

1

.

22

10

600

2

1

C

3

3

8

=

⋅

⋅

⋅

⋅

π

=













⋅

π

⋅

⋅

−

=

O

FB

O

C

S

CA

C

V

V

C

F

2

1

R

G

1

log

20

A

dB

9

.

15

3

.

3

0

.

1

10

22

10

80

2

1

10

1

.

6

28

1

log

20

A

6

3

3

C

=













⋅

⋅

⋅

⋅

⋅

π

⋅

⋅

⋅

⋅

−

=

−

−

m

7

g

10

R

20

C

A

=

CESAT

D

IN

D

O

V

V

V

V

V

D

−

+

+

=













−

=

1

V

0

.

1

V

R

R

O

6

4

1

SW

D

O

L

L

F

)

D

1

(

)

V

V

(

I

⋅

−

⋅

+

=

D

SW

O

D

O

1

F

I

%

20

)

D

1

(

)

V

V

(

L

⋅

⋅

−

⋅

+

=

)

D

1

(

D

I

I

O

CIN

_

RMS

−

⋅

⋅

=













⋅

⋅

+

⋅

D

=

D

O

SW

L

O

C

F

8

1

ESR

I

V

SW

IN

O

IN

F

V

4

I

C

⋅

D

⋅

>

,

R

G

R

G

S

CA

PWM

⋅

≈

)

/

s

Q

/

s

1

()

/

s

1

(

)

C

R

s

1

(

G

V

V

2

n

2

n

p

O

ESR

PWM

c

o

ω

+

ω

+

ω

+

+

=

7

1

Z

5

R

F

2

1

C

π

=

7

1

P

8

R

F

2

1

C

π

=

,

C

R

1

O

ω

,

C

R

1

O

ESR

ω

k

3

.

22

10

28

.

0

10

R

3

7

20

9

.

15

=

⋅

=

−

nF

45

.

0

10

1

.

22

10

16

2

1

C

3

3

5

=

⋅

⋅

⋅

⋅

π

=

pF

12

10

1

.

22

10

600

2

1

C

3

3

8

=

⋅

⋅

⋅

⋅

π

=













⋅

π

⋅

⋅

−

=

O

FB

O

C

S

CA

C

V

V

C

F

2

1

R

G

1

log

20

A

dB

9

.

15

3

.

3

0

.

1

10

22

10

80

2

1

10

1

.

6

28

1

log

20

A

6

3

3

C

=













⋅

⋅

⋅

⋅

⋅

π

⋅

⋅

⋅

⋅

−

=

−

−

m

7

g

10

R

20

C

A

=

whereV

voltage, and V

diode.

In peak current-mode control, the PWM modulating

ramp is the sensed current ramp of the power switch.

This current ramp is absent unless the switch is turned

on. The intersection of this ramp with the output of the

voltage feedback error amplifier determines the switch

pulse width. The propagation delay time required to

immediately turn off the switch after it is turned on is the

minimum controllable switch on time (T

Closed-loop measurement shows that the SC4525D

minimum on time is about 120ns at room temperature

(Figure 4). If the required switch on time is shorter than

the minimum on time, the regulator will either skip cycles

or it will jitter.

Figure 4. Variation of Minimum On Time

with Ambient Temperature

To allow for transient headroom, the minimum operating

switch on time should be at least 20% to 30% higher than

the worst-case minimum on time.

Minimum Off Time Limitation

The PWM latch in Figure 2 is reset every cycle by the

clock. The clock also turns off the power transistor to

refresh the bootstrap capacitor. This minimum off time

limits the attainable duty cycle of the regulator at a given

12

12

Fig.4: Min On-Time vs Temp

Minumum On Time vs Temperature

100

120

140

160

180

200

-50 -25

0

25

50

75 100 125

SS270 REV 6-7

12

12

Fig.4: Min On-Time vs Temp

Minumum On Time vs Temperature

100

120

140

160

180

200

-50 -25

0

25

50

75 100 125

SS270 REV 6-7