ISL97635A datasheet(26/29 Pages) RENESAS

26 / 29 page

ISL97635A

FN6564 Rev.3.00

Page 26 of 29

Sep 26, 2017

Applications

High Current Applications

Each channel of the ISL97635A 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 six channels of LEDs are

needed, multiple ISL97635As with each driver having its own

supporting components can be controlled together with the

common SMBus. While the ISL97635A does not have extra pins

strappable slave address feature, a 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 EN/PWMI signal is common to all drivers.

16-Bit Dimming

The SMBus controlled PWM and DC dimmings can be combined

to effectively provide 16 bits of dimming capability, which can be

valuable for automotive and avionics display applications.

Figure 33 illustrates one programming example where 256 steps

of PWM dimming can be programmed between each DC

dimming steps or vice versa.

RGB LED Backlight or Scrolling Backlight

Operation

The SMBus control features of PWM dimming, DC dimming, and

random channels selection have offered many driving

possibilities. For example, red, green, and blue LEDs can be

arranged in Ch0 and Ch1, Ch2 and Ch3, Ch4 and Ch5

respectively such that each group can be controlled

independently in sequential order for RGB LED backlighting

applications.

Compensation

The ISL97635A has two main elements in the system; the

Current Mode Boost Regulator and the op amp based

multi-channel current sources. The ISL97635A incorporates a

transconductance amplifier in its feedback path to allow the user

some levels of adjustment on the transient response and better

regulation. The ISL97635A uses current mode control

architecture, which has a fast current sense loop and a slow

voltage feedback loop. The fast current feedback loop does not

require any compensation. The slow voltage loop must be

compensated for stable operation. The compensation network is

a series Rc, Cc1 network from COMP pin to ground and an

optional Cc2 capacitor connected to the COMP pin. The Rc sets

the high frequency integrator gain for fast transient response and

the Cc1 sets the integrator zero to ensure loop stability. For most

applications, Rc is in the range of 200Ω to 3kΩ and Cc1 is in the

range of 27nF to 37nF. Depending upon the PCB layout, a Cc2, in

range of 100nF, may be needed to create a pole to cancel the

output capacitor ESR’s zero effect for stability. The ISL97635A

evaluation board is configured with Rc1 of 500Ω, Cc1 of 33nF,

and Cc2 of 0, which achieves stability. In the actual applications,

these values may need to be tuned empirically but the

recommended values are usually a good starting point.

FIGURE 31. GROUPING MULTIPLE CHANNELS FOR HIGH CURRENT

APPLICATIONS

IIN0

IIN1

IIN2

VOUT

FIGURE 32. MULTIPLE DRIVERS OPERATION

SMBCLK

SMBDAT

EN/PWMI

SMBCLK

SMBDAT

EN/PWMI

SMBCLK

SMBDAT

EN1

EN2

FIGURE 33. 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

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