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ISL976787IBZ-T

4-Channel LED Driver with Phase Shift Control and 10-Bit Dimming Resolution

Intersil 

ISL97687

Inductor

The selection of the inductor should be based on its maximum

current (ISAT) characteristics, power dissipation, EMI

susceptibility (shielded vs unshielded), and size. Inductor type

and value influence many key parameters, including the inductor

ripple current, current limit, efficiency, transient performance

and stability.

The inductor’s maximum current capability must be large enough

to handle the peak current at the worst case condition. If an

inductor core is chosen with a lower current rating, saturation in

the core will cause the effective inductor value to fall, leading to

an increase in peak to average current level, poor efficiency and

overheating in the core. The series resistance, DCR, within the

inductor causes conduction loss and heat dissipation. A shielded

inductor is usually more suitable for EMI susceptible

applications, such as LED backlighting.

The peak current can be derived from the voltage across the

inductor during the off period, as expressed in Equation 10:

ILpeak = (VO × IO ) ⁄ (85% × VI ) + 1 ⁄ 2[VI × (VO – VI ) ⁄ (L × VO × fSW ) ]

(EQ. 10)

The choice of 85% is just an average term for the efficiency

approximation. The first term is the average current, which is

inversely proportional to the input voltage. The second term is

the inductor current change, which is inversely proportional to L

and fSW. As a result, for a given switching frequency, minimum

input voltage must be used to calculate the input/inductor

current as shown in Equation 10. For a given inductor size, the

larger the inductance value, the higher the series resistance

because of the extra number of turns required, thus, higher

conductive losses. The ISL97687 current limit should be less

than the inductor saturation current.

Output Capacitors

The output capacitor acts to smooth the output voltage and

supplies load current directly during the conduction phase of the

power switch. Output ripple voltage consists of the discharge of

the output capacitor during the FET turn-on period and the

voltage drop due to load current flowing through the ESR of the

output capacitor. The ripple voltage is shown in Equation 11:

ΔVCO = (IO ⁄ CO × D ⁄ fSw ) + ((IO × ESR)

(EQ. 11)

where IO represents the output current, CO is the output

capacitance, D is the duty ratio as described in Equation 9. ESR

is the equivalent series resistance of the output capacitance and

fsw is the switching frequency of the converter. Equation 11

shows the importance of using a low ESR output capacitor for

minimizing output ripple.

As shown in Equation 11, the output ripple voltage, ΔVCo, can be

reduced by increasing the output capacitance, CO or the

switching frequency, fSW, or using output capacitors with small

ESR. In general, ceramic capacitors are the best choice for

output capacitors in small to medium sized LCD backlight

applications due to their cost, form factor, and low ESR.

The choice of X7R over Y5V ceramic capacitors is highly

recommended because the X7R type capacitor is less sensitive

to capacitance change overvoltage. Y5V’s absolute capacitance

can be reduced to 10%~20% of its rated capacitance at the

maximum voltage. In any case, Y5V type ceramic capacitors

should be avoided.

A larger output capacitor will also ease the driver response

during PWM dimming off period due to the longer sample and

hold effect of the output drooping. The driver does not need to

boost as much on the next on period, which minimizes transient

current. The output capacitor also plays an important role for

system compensation.

Channel Capacitor

It is recommended to use at least 1nF capacitors from CH pins to

VOUT. Larger capacitors will reduce LED current ripple at boost

frequency, but will degrade transient performance at high PWM

frequencies. The best value is dependant on PCB layout. Up to

4.7nF is sufficient for most configurations.

Schottky Diode

A high speed rectifier diode is necessary to prevent excessive

voltage overshoot, especially in the boost configuration. Low

forward voltage and reverse leakage current will minimize

losses, making Schottky diodes the preferred choice. Although

the Schottky diode turns on only during the boost switch off

period, it carries the same peak current as the inductor,

therefore, a suitable current rated Schottky diode must be used.

High Current Applications

Each channel of the ISL97687 can support up to 160mA. For

applications that need higher current, multiple channels can be

grouped to achieve the desirable current. For example, in

Figure 27, the cathodes of the last LEDs can be connected to

CH1/CH2 and CH3/CH4, this configuration can be treated as a

single string with up to 350mA current driving capability.

BOOST OUTPUT

CH1

CH2

CH3

CH4

FIGURE 27. GROUPING MULTIPLE CHANNELS FOR HIGH CURRENT

APPLICATIONS

17

FN7714.0

September 15, 2011

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