ISL97635A datasheet(12/29 Pages) RENESAS

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ISL97635A

FN6564 Rev.3.00

Page 12 of 29

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 ISL97635A 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 6 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 Register 0x01 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 (Applicable to SMBus controlled PWM,

DPST, and DC-to-PWM Modes)” 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 follows:

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 the external PWM signal.

The DPST mode PWM frequency is adjusted by a capacitor at the

FPWM pin. Also, a CPWMO capacitor is also needed, which will be

described in “PWM Dimming Frequency Adjustment (Applicable

to SMBus controlled PWM, DPST, and DC-to-PWM Modes)” on

page 13.

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

at 200Hz (see CFPWM in Equation 10) 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 using a

40kΩ resistor when the PWMI/EN 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 (Applicable to SMBus controlled

PWM, DPST, and DC-to-PWM Modes)” on page 13.

RSET

733 0.02



36.6k



(EQ. 3)

ILED

2.87200 36600



15.7mA

(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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