MAX8790(2006) Просмотр технического описания (PDF) - Maxim Integrated

Номер в каталоге

Компоненты Описание

производитель

MAX8790
(Rev.:2006)

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

Maxim Integrated 

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

Table 4. Component Suppliers

SUPPLIER

PHONE

WEBSITE

Murata

Nichia

Sumida

Toshiba

770-436-1300

248-352-6575

847-545-6700

949-455-2000

www.murata.com

www.nichia.com

www.sumida.com

www.toshiba.com/taec

Vishay

203-268-6261

www.vishay.com

components for the typical applications circuit. Table 4

lists component suppliers. External component value

choice is primarily dictated by the output voltage and the

maximum load current, as well as maximum and minimum

input voltages. Begin by selecting an inductor value.

Once L is known, choose the diode and capacitors.

Inductor Selection

The inductance, peak current rating, series resistance,

and physical size should all be considered when

selecting an inductor. These factors affect the conver-

ter’s operating 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 rip-

ple, 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

MAX8790 has a fixed internal slope compensation,

which requires a minimum inductor value. When CCM

mode is chosen, the ripple current and the peak cur-

rent 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 mini-

mized 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 require-

ment 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 cur-

rent. 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 converter 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 diameter. If the

inductor resistance is relatively low, increasing induc-

tance 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 typ-

ical input voltage (VIN), the maximum output current

(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 MAX8790 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:

( ) LCCM(MIN) =

VOUT(MAX) + VDIODE − 2 × VIN(MIN)

51mV × fOSC(MIN)

× RS

______________________________________________________________________________________ 17

Share Link: