ISL97635 datasheet(25/28 Pages) INTERSIL

25 / 28 page

FN6434.2

December 22, 2008

output capacitor. The ripple voltage can be shown as

Equation 21:

The conservation of charge principle also brings up the fact

that during the boost switch Off-period, the output capacitor

is charged with the inductor ripple current minus a relatively

small output current in boost topology. As a result, the user

needs to select an output capacitor with low ESD and

enough input ripple current capability.

Output Ripple

ΔV

Co, can be reduced by increasing Co or fSW, or using

small ESR capacitors. 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.

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 harder in the next On-period that minimizes

transient current. The output capacitor is also needed for

compensation, and, in general 2x4.7µF/50V ceramic

capacitors are suitable for notebook display backlight

applications.

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, and therefore, a suitable current

rated Schottky diode must be used.

Applications

High Current Applications

Each channel of the ISL97635 can support up to 35mA. For

applications that need higher current, multiple channels can

be grouped to achieve the desirable current. For example,

the cathode of the last LED can be connected to IIN0 to IIN2,

this configuration can be treated as a single string with

105mA current driving capability.

Multiple Drivers Operation

For large LCD panels where more than 8 channels of LEDs

are needed, multiple ISL97635s with each driver having its

own supporting components can be controlled together with

the common SMBus. While the ISL97635 does not have

extra pins strappable slave address feature, separate EN

signal can be applied to each driver for asynchronous

operation. A trade-off of such scheme is that an exact faulty

channel cannot be identified if the PWMI/EN signal is

common to all drivers.

ΔV

(

O CO

S )

(

ESR

()

⁄

(EQ. 21)

FIGURE 30. GROUPING MULTIPLE CHANNELS FOR HIGH

CURRENT APPLICATIONS

IIN0

IIN1

IIN2

VOUT

FIGURE 31. MULTIPLE DRIVERS OPERATION

SMBCLK

SMBDAT

EN/PWMI

SMBCLK

SMBDAT

EN/PWMI

SMBCLK

SMBDAT

EN1

EN2

FIGURE 32. 16-BIT DIMMING ILLUSTRATION

STEP 255 PWM CONTROL

STEP 0~255 DC CONTROL

STEP 254 PWM CONTROL

STEP 1 PWM CONTROL

STEPS 0~255 DC

STEP 0 PWM CONTROL

ISL97635

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