ML4895

6

Q (ONE SHOT)

INDUCTOR CURRENT (IL)

VSR

tON

VC

Figure 4. One Shot and Synchronous Rectifier Timing

DESIGN CONSIDERATIONS

A typical design can be implemented by using the

following design procedure. Note that this procedure is

not intended to give final values, but to give a good

starting point, and provide the relationships necessary to

make trade-off decisions. Some experimentation will be

necessary to optimize values and to verify that the design

operates over worst case conditions.

DESIGN SPECIFICATIONS

It is important to start with a clear definition of the design

specifications. Make sure the specifications reflect worst

case conditions. Key specifications include the minimum

and maximum input voltage and the output voltage and

load current.

INDUCTOR AND SENSE RESISTOR SELECTION

Figure 5 shows the inductor current of the buck regulator.

The inductor current is made up of two components: the

DC current level set by the transconductance amplifier,

II

I

V

R

TV

V

L

OUT

SENSE

L

SENSE

SENSE

ON

IN

OUT

=+

=

+

×−

×

1

22

bg

(3)

Therefore, the selection of the inductance value

determines how much of the output current is made up of

the ripple current. Higher inductor ripple current allows

smaller inductor values, but results in higher peak

currents, lower efficiency, and higher output voltage

ripple.

Inductor ripple currents in the range of 30% to 70% of the

maximum output current are typical. As a good starting

point, set the inductor ripple current to 50% of the

maximum output current:

∆I

TV

V

L

FI

L

ON

IN

OUT

IRC

OUT MAX

=

×−

=×

bg

()

(4)

where FIRC = ratio of inductor ripple current to the

maximum output current, or:

L

TV

V

I

ON

IN

OUT

OUT MAX

=

×−

×

bg

05

.

()

(5)

Calculate the inductance using the volt-seconds value

given in Figure 3 at the maximum input voltage. Choose

the nearest standard value, realizing the trade-offs

mentioned before. Then, using the inductance value

chosen, determine the actual inductor ripple current at

the maximum and minimum input voltage using Equation

4 and Figure 3.

The sense resistor value can be determined using the

inductor ripple current value calculated above and

Equation 3 rearranged as follows:

R

V

II

SENSE

SENSE MIN

OUT MAX

L MIN

=

−

()

()

(

)

1

2

∆

(6)

Having determined the values for the inductor and sense

resistor, we can now specify the inductor peak current

rating. This value is calculated at current limit and at the

maximum input voltage, and is given by:

II

I

L PEAK MAX

SENSE MAX

L MAX

(

(

))

()

()

=+ ∆

(7)

I

V

R

I

LPEAK MAX

SENSE MAX

SENSE

LMAX

af

ch

af

af

=+∆

I

V

R

I

LPEAK MAX

SENSE

LMAX

af

ch

af

=+

01

.

∆