where 51mV is a scale factor based on slope compen-

temporarily calculated using the following equation:

where 100mV is the current-limit sense voltage.

The minimum inductor value should be recalculated

Selection section).

Choose an available inductor value from an appropriate

inductor family. Calculate the maximum DC input cur-

servation of energy and the expected efficiency at that

operating point (

η

in the

Typical Operating Characteristics:

Calculate the ripple current at that operating point and

the peak current required for the inductor:

When DCM operating mode is chosen to minimize the

inductor value, the calculations are different from that in

the above CCM mode. The maximum inductor value for

DCM mode is calculated by the following equation:

The peak inductor current in DCM mode is calculated

using the following equation:

The inductor’s saturation current rating should exceed

inductor with less than 0.1Ω series resistance.

Considering the typical operating circuit, the maximum

and a minimal input voltage of 7V. Choosing a DCM

operating mode and estimating efficiency of 90% at this

operating point:

4.7µH inductor is chosen. The peak inductor current at

minimum input voltage is calculated as follows:

Sense-Resistor Selection

The detected signal is fed into the step-up converter

control compensation loop through the CS pin.

The MAX8790’s current-mode step-up converter senses

the switch current from CS to GND with an external

100mV. The required resistance is calculated based

upon the peak inductor current at the end of the switch

on-time:

where 25.6mV is a scale factor from slope compensa-

the maximum duty cycle at minimum input voltage and

maximum output voltage. In DCM operating mode, it is

calculated by the following equation:

For the typical operating circuit as Figure 1:

rent-sense resistor is chosen.

R

mV

mV

A

m

S

<

+×

−

()

=

85

25 6

0 75 0 68

135

64

..

.

.

Ω

D

H

A

MHz

V

MAX

=

××

=

47

135

0 75

7

068

..

.

.

μ

D

LI

f

V

MAX

LIM

OSC

IN MIN

= ××

()

R

VmV

D

I

S

CS EC

MAX

PEAK

<

+×

−

()

_

..

25 6

0 75

I

mA

V

V

V

V

H

MHz

V

V

A

PEAK

=

××

×

+

−

()

μ×

×

×

+

()

=

120

2 2872

2872

0 4

7

4 7

0 675

0 9

28 72

0 4

135

..

.

..

.

.

.

.

L

V

VV

V

MHz

V

mA

H

DCM MAX

()

..

()

.

..

.

=−

+

⎛

⎝⎜

⎞

⎠⎟

×

×

××

×

=

1

7

28 72

0 4

70 9

2 0 825

28 72

120

58

2

μ

I

IV

V

V

V

Lf

V

V

PEAK

OUT

OUT MAX

OUT MAX

DIODE

IN MIN

OSC MIN

OUT MAX

DIODE

=

××

×

+

−

()

×× ×

+

()

(max)

(

)

(

)

(

)

()

(

)

2

η

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

II

I

PEAK

IN DCMAX

RIPPLE

=+

(,

)

2

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

R

mV

I

S TMP

IN DCMAX

_

(,

)

.

=

×

100

12

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

18

______________________________________________________________________________________