down switching regulator in continuous-conduction

mode (CCM) is given by

(2)

where V

voltage drop across the rectifying 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 SC4524A

minimum on time is about 35ns 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.

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

=

Fig.4

Minimum On Time vs Temperature

100

110

120

130

140

150

160

170

180

190

200

-50 -25

0

25

50

75 100 125

SS270 REV 6-7

1MHz

Fig.4

Minimum On Time vs Temperature

100

110

120

130

140

150

160

170

180

190

200

-50 -25

0

25

50

75 100 125

SS270 REV 6-7

1MHz

SC4524A

0

Applications Information (Cont.)

As the load draws more current from 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

soft-start capacitor is discharged with (I

the inductor current falls below the current limit and the

PWM comparator instead turns off the switch, then latch

B

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 .9V. At this

juncture, comparator B

soft-start capacitor will be continuously discharged with

(I

switching regulator is shut off until the soft-start capacitor

is discharged below .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 is set with an external

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

FB pin. For a given R

()

Setting the Switching Frequency

The switching frequency of the SC4524A is set with an

external resistor from the ROSC pin to ground.

Minimum On Time Consideration

The operating duty cycle of a non-synchronous step-

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

=