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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

Input Capacitor Selection

The input capacitor (CIN) filters the current peaks drawn

from the input supply and reduces noise injection into

the IC. A 10µF ceramic capacitor is used in the typical

operating circuit (Figure 1) because of the high source

impedance seen in typical lab setups. Actual applica-

tions usually have much lower source impe-dance since

the step-up regulator often runs directly from the output

of another regulated supply. In some applications, CIN

can be reduced below the values used in the typical

operating circuit. Ensure a low noise supply at IN by

using adequate CIN. Alternatively, greater voltage varia-

tion can be tolerated on CIN if IN is decoupled from CIN

using an RC lowpass filter.

Select CIN’s RMS ripple current rating to ensure that its

thermal rise is less than approximately 10°C:

IRMS = dIL

2× 3

LED Selection and Bias

The series/parallel configuration of the LED load and the

full-scale bias current have a significant effect on regu-

lator performance. LED characteristics vary significantly

from manufacturer to manufacturer. Consult the respec-

tive LED data sheets to determine the range of output

voltages for a given brightness and LED current. In ge-

neral, brightness increases as a function of bias current.

This suggests that the number of LEDs could be

decreased if higher bias current is chosen; however,

high current increases LED temperature and reduces

operating life. Improvements in LED technology are

resulting in devices with lower forward voltage while

increasing the bias current and light output.

LED manufacturers specify LED color at a given LED

current. With lower LED current, the color of the emitted

light tends to shift toward the blue range of the spec-

trum. A blue bias is often acceptable for business appli-

cations but not for high-image-quality applications such

as DVD players. Direct DPWM dimming is a viable solu-

tion for reducing power dissipation while maintaining

LED color integrity. Careful attention should be paid to

switching noise to avoid other display quality problems.

Using fewer LEDs in a string improves step-up

converter efficiency, and lowers breakdown voltage

requirements of the external MOSFET and diode. The

minimum number of LEDs in series should always be

greater than the maximum input voltage. If the diode

voltage drop is lower than the maximum input voltage,

the voltage drop across the current-sense inputs (FB_)

increases and causes excess heating in the IC.

Between 8 and 12 LEDs in series is ideal for input volt-

ages up to 20V.

Applications Information

LED VFB_ Variation

The MAX8790 has accurate (±1.5%) matching for each

current source. However, the forward voltage of each

white LED can vary up to ±5% from part to part. The

accumulated voltage difference in each string equates

to additional power loss within the IC. For the best effi-

ciency, the voltage difference between strings should

be minimized. The difference between lowest voltage

string and highest voltage string should be less than

4.5V. Otherwise, the internal LED short-circuit protection

shuts the part off.

Choosing the Appropriate Dimming Mode

Analog dimming mode allows lower peak LED current

and results in higher converter efficiency and lower

noise compared to direct DPWM mode. Unfortunately,

the LED color spectrum can shift as a function of DC

current so DPWM mode is often used to achieve more

consistent display characteristics. (See the LED manu-

facturer’s data sheet to determine the extent of the

color shift.) When the MAX8790 is configured with an

FSET resistor and CPLL capacitor, the ENA signal can

toggle between modes on the fly. Care should be exer-

cised when switching between modes to prevent the

current from becoming unstable during the PLL lock-in

time. To avoid such problems, force the controller into

DPWM mode between transitions.

LCD Panel Capacitance

Some LCD panels include a capacitor in parallel with

LED string to improve ESD immunity. Because of the

10µA pullup current source in each FB_ input for string

detection, the MAX8790 can start up with less than

470pF capacitance on each FB_ pin. If the string

capacitance CLED is greater than 470pF, a bank of

pullup resistors to VIN should be added to prevent

startup problems (see Figure 6). A delay of 3 x 1MΩ x

CLED should be added after VIN was settled before

enabling the MAX8790 to ensure the FB_ voltage

exceeds the 3V internal threshold. A similar delay

should be added after the part is shut down to ensure

proper restart.

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