The MAX17061A has a minimum inductor value limita-

tion for a stable operation in CCM mode at low input

voltage because of the internal fixed-slope compensa-

tion. The minimum inductor value for stability is calcu-

lated with the following equation:

where 24.7mV is a scale factor from slope compensa-

factor (12m

Ω typ):

1) Choose an available inductor value from an appropri-

ate inductor family. Calculate the maximum DC input

conservation of energy and the expected efficiency

at that operating point (

ate curve in the

Typical Operating Characteristics:

2) Calculate the ripple current at that operating point

and the peak current required for the inductor:

Inductor Selection in DCM Operation

When DCM operating mode is chosen to minimize the

inductor value, the calculations are different from those

above in CCM mode. The maximum inductor value for

DCM mode is calculated with the following equation:

The peak-inductor current in DCM is calculated with fol-

lowing equation:

The inductor’s saturation current rating should exceed

inductor with less than 0.1

Ω series resistance.

Inductor Selection Design

Examples:

Considering the

Typical Operating Circuit with four

10-LED strings and 25mA LED full-scale current, the

35.9V output and a minimal input voltage of 7V.

Choosing a CCM operating mode with LIR = 1 at 1MHz

and estimating efficiency of 85% at this operating point:

A10µH inductor is chosen, which is higher than the

minimum L that guarantees stability in CCM.

The peak-inductor current at minimum input voltage is

calculated as follows:

Alternatively, choosing a DCM operating mode at 750kHz

and estimating efficiency of 85% at this operating point:

A 4.7µH inductor is chosen. The peak inductor current

at minimum input voltage is calculated as follows:

Output Capacitor Selection

The total output voltage ripple has two components: the

capacitive ripple caused by the charging and discharging

on the output capacitor, and the ohmic ripple due to the

capacitor’s equivalent series resistance (ESR):

and:

V

I

C

VV

Vf

RIPPLE C

OUT MAX

OUT

OUT MAX

IN MIN

OUT MAX OSC

()

()

()

(

)

()

≈

−

⎛

⎝

⎜

⎞

⎠

⎟

VV

V

RIPPLE

RIPPLE C

RIPPLE ESR

=+

()

(

)

I

mA

V

V

V

V

uH

MHz

V

V

A

××

×

+

−

()

××

×

+

()

=

100

2 35 9

35 9

0 4

7

47

0675

085

359

04

147

..

.

..

.

.

.

.

L

V

VV

V

MHz

V

mA

H

DCM MAX

()

..

()

.

..

.

=−

+

⎛

⎝⎜

⎞

⎠⎟

×

×

××

×

=

1

7

35 9

0 4

70 85

2 0 825

35 9

100

56

2

μ

I

mA

V

V

VV

V

H

V

MHz

A

×

×

+

×−

()

××

×

=

100

35 9

70 85

735 9

7

210

35 9

0 9

092

.

.

.

..

.

μ

L

VV

V

m

mV

MHz

H

CCM MIN

()

..

..

.

=

+− ×

××

=

35 9

0 4

2 7

12

224 7

0 9

60

Ω

μ

L

V

V

VV

mA

MHz

H

= ⎛

⎝⎜

⎞

⎠⎟

−

×

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

=

7

35 9

35 9

7

100

1

085

1

944

2

.

..

.

μ

II

I

PEAK

IN DCMAX

RIPPLE

=+

(,

)

2

L

V

VV

V

fV

I

DCM MAX

IN MIN

OUT MAX

DIODE

IN MIN

OSC MAX

OUT MAX

OUT MAX

()

()

()

()

()

()

()

=−

+

⎛

⎝

⎜

⎞

⎠

⎟

×

×

××

×

1

2

2

η

I

VV

V

LV

f

RIPPLE

IN MIN

OUT MAX

IN MIN

OUT MAX

OSC

=

×−

()

××

()

(

)

()

()

I

IV

V

IN DCMAX

OUT MAX

OUT

IN MIN

MIN

(,

)

()

()

=

×

×η

L

VV

V

R

mV f

CCM MIN

OUT MAX

DIODE

IN MIN

S

OSC MIN

()

()

(

)

()

.

=

+− ×

××

2

224 7

8-String White LED Driver with

SMBus for LCD Panel Applications

______________________________________________________________________________________

21