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Download or read online National Semiconductor LM3524D Regulating Pulse Width Modulator pdf datasheet.

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Typical Applications

(Continued)

V

T

V

t

L

IN

ON

From I

I

a

j

e

L

L

L

L1

(V

V

) t

b

o

IN

OFF

and I

b

j

L

L1

Since

I

a

I

b

V

t

V

t

e

e

b

L

L

IN

ON

o

OFF

and neglecting V

and V

SAT

D1

t

ON

V

j V

1

a

o

IN

t

OFF

The above equation shows the relationship between V

and duty cycle

In calculating input current I

which equals the induc-

IN(DC)

tor’s DC current assume first 100% efficiency

P

I

V

e

IN

IN(DC)

IN

t

ON

P

I

V

I

V

1

e

e

a

OUT

o

o

o

IN

t

OFF

for

100% P

P

e

e

OUT

IN

t

ON

I

V

1

I

V

a

e

o

IN

IN(DC)

t

OFF

t

ON

I

I

1

e

a

IN(DC)

o

t

OFF

This equation shows that the input or inductor current is

larger than the output current by the factor (1

Since this factor is the same as the relation between V

V

I

can also be expressed as

IN

IN(DC)

V

o

I

I

e

IN(DC)

o

V

IN

So far it is assumed

100% where the actual efficiency

e

or

will be somewhat less due to the saturation voltage

MAX

of Q1 and forward on voltage of D1 The internal power loss

due to these voltages is the average I

current flowing or

L

I

through either V

or V

For V

e

IN

SAT

D1

SAT

power loss becomes I

(1V)

is then

IN(DC)

MAX

P

V

I

o

o

o

e

e

e

MAX

P

V

I

I

(1V)

a

IN

o

o

IN

V

I

I

a

o

o

From V

V

1

e

a

o

IN

V

t

IN

OFF

This equation assumes only DC losses however

further decreased because of the switching time of Q1 and

D1

In calculating the output capacitor C

supplies I

during t

o

ON

V

IN

o

time will be some

regulator Calculation of C

V

o

T

From V

V

e

o

IN

t

OFF

where T

t

t

e

a

ON

OFF

V

IN

t

T

e

b

ON

V

I

T

o

C

e

o

t

t

)

a

ON

OFF

and

o

where C

is in farads f is the switching frequency

o

V

is the p-p output ripple

o

Calculation of inductor L1 is as follows

L1

e

V

is applied across L1

IN

I

0 4 I

e

e

Lp-p

L

V

t

V

1V this

e

IN

ON

D1

L1

e

V

0 4 I

o

V

I

V

o

o

t

ON

1

a

o

t

L1

OFF

where L1 is in henrys f is the switching frequency in Hz

15

t

ON

t

OFF

V

IN

e

max

V

1

a

IN

is

MAX

it can be seen that C

o

o

The voltage change on C

during this

o

V

V

or the output ripple of the

e

c

o

is

o

I

t

I

t

o

ON

o

ON

or C

e

e

o

C

V

o

o

V

IN

t

T

e

OFF

V

o

1

e

f

V

V

b

IN

o

IN

T

T

therefore

e

V

o

o

V

V

b

o

IN

V

I

(V

V

)

b

o

o

o

IN

e

V

f V

V

o

o

o

V

t

IN

ON

since during t

ON

I

a

L

V

o

0 41 I

0 4 I

therefore

e

IN

o

V

IN

T (V

V

)

b

o

IN

and since t

e

ON

V

o

o

IN

IN 2

2 5 V

(V

V

)

b

o

IN

e

o 2

f I

V

o

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