ISL97635 datasheet(12/28 Pages) INTERSIL

12 / 28 page

FN6434.2

December 22, 2008

MAXIMUM DC CURRENT SETTING

The initial brightness should be set by choosing an

appropriate value for RSET. This should be chosen to fix the

maximum possible LED current, as shown in Equation 1:

DC CURRENT ADJUSTMENT

Once RSET is fixed, the LED DC current can be adjusted

through Register 0x07 (BRTDC), as shown in Equation 2:

BRTDC can be programmed from 0 to 255 in decimal and

defaults to 255 (0xFF). If left at the default value, LED

current will be fixed at ILEDmax. BRTDC can be adjusted

dynamically on the fly during operation. BRTDC = 0

disconnects all channels and ILED is guaranteed to be <10µA

at this state.

For example, if the maximum required LED current (ILEDmax)

is 20mA, rearranging Equation 1 yields Equation 3:

If BRTDC is set to 200 then:

PWM CONTROL

The ISL97635 provides four different PWM dimming

methods, as described in the following. Each of these

methods results in PWM chopping of the current in the LEDs

for all 8 channels to provide an average LED current. During

the On periods, the LED current will be defined by the value

of RSET and BRTDC, as described in Equations 1 and 2. The

source of the PWM signal can be described as follows:

1. Internally generated 256 step duty cycle programmed

through the SMBus.

2. External signal from PWMI.

3. DPST mode. Internally generated signal with a duty cycle

defined by the product of the external PWMI and SMBus

programmed PWM at the internal setting frequency.

4. DC-to-PWM control.

The default PWM dimming is in DPST mode. In all four

methods, the average LED current of each channel is

controlled by ILED and the PWM duty cycle in percent, as

shown in Equation 5:

Method 1 (Internal Mode, SMBus controlled PWM)

The average LED current of each channel is controlled by the

internally generated PWM signal, as shown in Equation 6:

where BRT is the PWM brightness level programmed in the

defaults to 255 (0xFF). BRT = 0 disconnects all channels

and ILED is guaranteed to be <10µA in this state.

To use only the SMBus controlled PWM brightness control,

users need to set Register0x01 to 0x05 with EN/PWMI in

logic high.

The SMBus controlled PWM frequency is adjusted by a

capacitor at the FPWM pin, which will be described in “PWM

Dimming Frequency Adjustment” on page 13.

Method 2 (External Mode)

The average LED current of each channel can also be

controlled by an external PWMI signal, as shown in Equation 7:

The PWM dimming frequency can be for example 20kHz but

there are a minimum on and off time requirements such that

the dimming will be in the range of 10% to 99.5%. If the

dimming frequency is below 5kHz, the dimming range can

be 1% to 99.5%. The PWM dimming off time cannot be

longer than 28ms or else the driver will enter shutdown.

To use PWMI only brightness control, users need to set

Method 3 (DPST Mode)

The average LED current of each channel can also be

controlled by the product of the SMBus controlled PWM and

the external PWMI signals as:

Where:

Therefore:

Where BRT is the value held in Register 0x00 (default

setting 0xFF) controlled by SMBus and PWMI is the duty

cycle of the incoming PWMI signal. In this way, the users can

change the PWM current in ratiometric manner to achieve

DPST compliance backlight dimming.

To use the DPST mode, users need to set Register 0x01 to

0x01 with external PWM signal.

The DPST mode PWM frequency is adjusted by a capacitor at

the FPWM pin. A CPWMO capacitor, is also needed in the

LEDmax

733

SET

---------------

(EQ. 1)

LED

2.87

BRTDC R

SET

⁄

(EQ. 2)

SET

733 0.02

⁄

36.6k

(EQ. 3)

LED

2.87

200 36600

15.4mA

⁄

•

(EQ. 4)

LED ave

()

LED

PWM

(EQ. 5)

LED ave

()

LED

BRT 255

⁄

()

(EQ. 6)

LED ave

()

LED

PWMI

(EQ. 7)

LED ave

()

LEDxPWMDPST

(EQ. 8)

PWM

DPST

BRT 255

⁄

PWMI

(EQ. 9)

LED ave

()

LED

BRT 255

⁄

PWMI

(EQ. 10)

ISL97635

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