MAX8790A データシートの表示（PDF） - Maxim Integrated

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MAX8790A

Six-String White LED Driver with Active Current Balancing for LCD Panel Applications

Maxim Integrated 

MAX8790A

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

Table 4. Component Suppliers

Murata

Nichia

Sumida

Toshiba

Vishay

SUPPLIER

PHONE

770-436-1300

248-352-6575

847-545-6700

949-455-2000

203-268-6261

WEBSITE

www.murata.com

www.nichia.com

www.sumida.com

www.toshiba.com/taec

www.vishay.com

Inductor Selection

The inductance, peak current rating, series resistance,

and physical size should all be considered when selecting

an inductor. These factors affect the converter’s operat-

ing mode, efficiency, maximum output load capability,

transient response time, output voltage ripple, and cost.

The maximum output current, input voltage, output volt-

age, and switching frequency determine the inductor

value. Very high inductance minimizes the current ripple,

and therefore reduces the peak current, which decreases

core losses in the inductor and I2R losses in the entire

power path. However, large inductor values also require

more energy storage and more turns of wire, which

increases physical size and I2R copper losses in the

inductor. Low inductor values decrease the physical size,

but increase the current ripple and peak current. Finding

the best inductor involves the compromises among circuit

efficiency, inductor size, and cost.

When choosing an inductor, the first step is to deter-

mine the operating mode: continuous conduction mode

(CCM) or discontinuous conduction mode (DCM). The

MAX8790A has a fixed internal slope compensation,

which requires a minimum inductor value. When CCM

mode is chosen, the ripple current and the peak current

of the inductor can be minimized. If a small-size inductor

is required, DCM mode can be chosen. In DCM mode,

the inductor value and size can be minimized but the

inductor ripple current and peak current are higher than

those in CCM. The controller can be stable, independent

of the internal slope compensation mode, but there is a

maximum inductor value requirement to ensure the DCM

operating mode.

The equations used here include a constant LIR, which is

the ratio of the inductor peak-to-peak ripple current to the

average DC inductor current at the full-load current. The

controller operates in DCM mode when LIR is higher than

2.0, and it switches to CCM mode when LIR is lower than

2.0. The best trade-off between inductor size and con-

verter efficiency for step-up regulators generally has an

LIR between 0.3 and 0.5. However, depending on the AC

characteristics of the inductor core material and ratio of

inductor resistance to other power-path resistances, the

best LIR can shift up or down. If the inductor resistance

is relatively high, more ripple can be accepted to reduce

the number of required turns and increase the wire diam-

eter. If the inductor resistance is relatively low, increasing

inductance to lower the peak current can reduce losses

throughout the power path. If extremely thin high-resis-

tance inductors are used, as is common for LCD panel

applications, LIR higher than 2.0 can be chosen for DCM

operating mode.

Once a physical inductor is chosen, higher and lower

values of the inductor should be evaluated for efficiency

improvements in typical operating regions. The detail

design procedure can be described as follows:

Calculate the approximate inductor value using the

typical input voltage (VIN), the maximum output cur-

rent (IOUT(MAX)), the expected efficiency (ηTYP) taken

from an appropriate curve in the Typical Operating

Characteristics, and an estimate of LIR based on the

above discussion:

L

=





VIN_MIN

VOUT





2



VOUT − VIN_MIN

IOUT(MAX) × fOSC



η TYP

LIR





The MAX8790A has a minimum inductor value limitation

for stable operation in CCM mode at low input voltage

because of the internal fixed slope compensation. The

minimum inductor value for stability is calculated by the

following equation:

( ) L CCM(MIN) =

VOUT(MAX) + VDIODE − 2 × VIN(MIN)

51mV × fOSC(MIN)

×RS

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

tion, and RS is the current-sense resistor. To determine

the minimum inductor value, the RS can be temporarily

calculated using the following equation:

R S _TMP

=

100mV

1.2 × IIN(DC,MAX)

where 100mV is the current-limit sense voltage.

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