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ISL97635
I LEDmax = ---------------
R SET
I LED ( ave ) = I LED × ( BRT ? 255 )
MAXIMUM DC CURRENT SETTING
The initial brightness should be set by choosing an
appropriate value for R SET . This should be chosen to fix the
maximum possible LED current, as shown in Equation 1:
733
(EQ. 1)
DC CURRENT ADJUSTMENT
Once R SET is fixed, the LED DC current can be adjusted
through Register 0x07 (BRTDC), as shown in Equation 2:
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:
(EQ. 6)
where BRT is the PWM brightness level programmed in the
Register 0x00. BRT ranges from 0 to 255 in decimal and
defaults to 255 (0xFF). BRT = 0 disconnects all channels
and I LED is guaranteed to be <10μA in this state.
I LED = 2.87 × BRTDC ? R SET
(EQ. 2)
To use only the SMBus controlled PWM brightness control,
users need to set Register0x01 to 0x05 with EN/PWMI in
logic high.
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 I LEDmax . BRTDC can be adjusted
dynamically on the fly during operation. BRTDC = 0
disconnects all channels and I LED is guaranteed to be <10μA
at this state.
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:
For example, if the maximum required LED current (I LEDmax )
is 20mA, rearranging Equation 1 yields Equation 3:
I LED ( ave ) = I LED × PWMI
(EQ. 7)
R SET = 733 ? 0.02 = 36.6k Ω
If BRTDC is set to 200 then:
I LED = 2.87 ? 200 ? 36600 = 15.4mA
PWM CONTROL
The ISL97635 provides four different PWM dimming
(EQ. 3)
(EQ. 4)
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.
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 R SET and BRTDC, as described in Equations 1 and 2. The
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:
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.
I LED ( ave ) = I LED xPWM DPST
Where:
PWM DPST = BRT ? 255 × PWMI
Therefore:
I LED ( ave ) = I LED × BRT ? 255 × PWMI
(EQ. 8)
(EQ. 9)
(EQ. 10)
The default PWM dimming is in DPST mode. In all four
methods, the average LED current of each channel is
controlled by I LED and the PWM duty cycle in percent, as
shown in Equation 5:
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
I LED ( ave ) = I LED × PWM
12
(EQ. 5)
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 C PWMO capacitor, is also needed in the
FN6434.2
December 22, 2008
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