ML4880

10

The relationship of the inductor current to the output

current is given by the following equation:

II

I

V

VV

V

R

TV

L

V

VV

OUT

SENSE

L

IN

IN

OUT

SENSE

SENSE

ON

IN

IN

IN

OUT

=+









+









=+

×

×









+









1

22

∆

(18)

The inductance value can be determined by setting the

ripple current portion of Equation 18 to some percentage

of the output current. Inductor ripple currents in the range

of 50% to 80% of the maximum output current are

typical. As a good starting point set the inductor ripple

current to 65% of the maximum output current:

∆I

TV

L

FI

VV

V

L

ON

IN

IRC

OUT MAX

IN

OUT

IN

=

×

=×

×

+









()

(19)

maximum output current, or:

L

TV

I

V

VV

ON

IN MIN

OUT MAX

IN MIN

IN MIN

OUT

=

×

×+









()

()

()

()

.

065

(20)

Calculate the inductance using the volt-seconds value

given in Figure 5 at the minimum input voltage. Choose

the nearest standard value, and determine the actual

inductor ripple current at the maximum and minimum

input voltage using Equation 19 and Figure 5.

Next, determine the sense resistor value using the inductor

ripple current value calculated above using:

R

V

I

VV

V

I

I

VV

V

I

SENSE

SENSE MIN

OUT MAX

IN MIN

OUT

IN MIN

L MIN

OUT MAX

IN MIN

OUT

IN MIN

L MIN

=

+









−

=

+









−

()

()

()

()

()

()

()

()

()

.

1

2

014

1

2

∆

∆

(21)

Having determined the values for the inductor and sense

resistor, the inductor peak current rating can now be

specified. This value is calculated at current limit and at

the maximum input voltage, and is given by:

II

I

V

R

I

V

R

I

L PK MAX

SENSE MAX

L MAX

SENSE MAX

SENSE

L MAX

SENSE

L MAX

(@

)

(

)

(

)

()

()

()

.

=+

=+

=

+

∆

∆∆

025

(22)

For reliable operation, the inductor current rating should

exceed the value calculated by 10%-20%.

Now the sense resistor’s power rating can be determined.

The sense resistor must be able to carry the peak current

in the inductor, but only during the OFF-time:

PI

R

R

RMS OFF

SENSE

SENSE

=×

()

2

(23)

where,

I

V

VV

II

I

I

RMS OFF

IN MAX

IN MAX

OUT

SENSE MAX

SENSE MAX

L PK MAX

L PK MAX

()

()

()

()

()

(

@

)

(

@

)

()

() (

) (

)

2

22

3

=

+









++









The winding resistance and the turns ratio of the coupled

inductor need to be specified. In general the winding

resistance should be as low as possible, preferably in the

low milliohm range. A good rule of thumb is to allow 2

mW of winding resistance per mH of inductance. The

turns ratio is specified at 1:1 for simplicity and availability.

FLYBACK REGULATOR - MOSFET SELECTION

The internal gate drive of the ML4880 limits the selection

of the switching FET to logic level types with the ON

with a maximum total gate charge of around 70nC.

The drain to source breakdown voltage rating is

determined as follows:

VV

V

F

DS

IN MAX

OUT

DS

=+

×

()

()

(24)

recommended due to the voltage spikes caused by the

leakage inductance of a transformer. For a more reliable

design, look for MOSFETs that are avalanche rated.

The maximum ON resistance of the MOSFET can be

determined using the worst case power dissipation for the

package type chosen. This assumes that the major portion

R

P

I

DS ON

PKG

RMS ON

()

()

=

2

(25)

where,

I

V

VV

II

I

I

RMS ON

OUT

IN MIN

OUT

SENSE MAX

SENSE MAX

L PK

MAX

L PK

MAX

()

()

()

()

(

@

)

(@

)

(

)

(

) () ()

2

2

2

3

=

+









++









FLYBACK REGULATOR — RECTIFIER SELECTION

The output diode must be able to carry the worst case

output current as well as withstand the worst case reverse

voltage. The worst case output current occurs at current

limit and at the maximum input voltage. Therefore, the

average forward current rating of the output diode can be

determined by the following:

I

I

FR

TV

L

V

VV

F

D AVG

OUT MAX

ID

SENSE

ON

IN MAX

IN MAX

IN MAX

OUT

ID

()

()

()

()

()

.

==

+

×

×









+

















025

2

1

where,

F

derating factor

(26)

The derating factor is typically in the range of 0.5 to 0.9

depending on reliability goals. Typically a value of 0.8 is

used for most applications.

The reverse voltage rating is determined by the sum of the

output voltage and the maximum input voltage: