ISL97635IRZ datasheet(12/28 Pages) RENESAS

12 / 28 page

ISL97635

FN6434 Rev.3.00

Page 12 of 28

Sep 26, 2017

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

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 Register

0x01 to 0x03.

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 PWMO pin for

DPST mode operation which will be described in “PWM Dimming

Frequency Adjustment” on page 13.

For example, if the SMBus controlled PWM duty is 80% dimming

at 200Hz (see CFPWM Equation 12) and the external PWMI duty

cycle is 60% dimming at 1kHz, the resultant PWM duty cycle is

48% dimming at 200Hz.

Method 4 (Analog Mode, DC-to-PWM Mode)

By overdriving the PWMO pin with a DC voltage between 0.21V

and 1.21V, the average LED current of each channel is controlled

by the internally generated PWM signal, as shown in

Equation 11:

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

0xFF). The PWMO pin is internally driven to 0.21V via a

40kΩ resistor when the EN/PWMI pin is in logic high, any

overdrive circuit will need to be able to drive up to 40µA in order

to overcome this.

The DC-to-PWM controlled PWM frequency is adjusted by a

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

Dimming Frequency Adjustment” on page 13.

For example, if PWMO is applied with a DC voltage

1.21V, the

output will be zero. On the other hand, if the PWMO is applied

with a DC voltage

0.21V, the PWM duty cycle will be at its

maximum. If the PWMO pin is applied with a DC voltage of 0.31V,

the PWM duty cycle will be at 90% at 200Hz if CFPWM = 27nF.

RSET

733 0.02



36.6k



(EQ. 3)

ILED

2.87 200 36600

15.4mA





(EQ. 4)

ILED ave



ILED PWM



(EQ. 5)

ILED ave



ILED BRT 255







(EQ. 6)

ILED ave



ILED PWMI



(EQ. 7)

ILED ave



ILEDxPWMDPST

(EQ. 8)

PWMDPST

BRT 255



PWMI



(EQ. 9)

ILED ave



ILED BRT 255





PWMI



(EQ. 10)

ILED ave



ILED BRT 255 1 V PWMO

 0.21

–



–









(EQ. 11)

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