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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

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 applications

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 variation can be toler-

ated 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 regula-

tor 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

general, brightness increases as a function of bias cur-

rent. This suggests that the number of LEDs could be

decreased if higher bias current is chosen; however, high

current increases LED temperature and reduces operat-

ing 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 spectrum.

A blue bias is often acceptable for business applications

but not for high-image-quality applications such as DVD

players. Direct DPWM dimming is a viable solution 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 maxi-

mum 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 voltages up to 20V.

Applications Information

LED VFB_ Variation

The MAX8790A 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 accu-

mulated voltage difference in each string equates to addi-

tional power loss within the IC. For the best efficiency, 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 manufacturer’s data

sheet to determine the extent of the color shift.) When the

MAX8790A is configured with an FSET resistor and CPLL

capacitor, the ENA signal can toggle between modes on

the fly. Care should be exercised 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. The MAX8790A can start

up without a problem for string capacitance up to 0.27μF.

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