ISL97673

FN7633 Rev.3.00

Page 13 of 28

Sep 19, 2017

Method 2 (External applied PWM)

To use this mode users need to set Register 0x01 to 0x03

The average LED current of each channel can also be

controlled by an external PWM signal as Equation 8:

PWM Dimming Frequency Adjustment

(Applicable to SMBus/I2C controlled PWM

and DPST Modes)

Except for the external PWM dimming mode, the

dimming frequencies of any other modes are set by an

external resistor at the FPWM pin as Equation 9:

where FPWM is the desirable PWM dimming frequency and

RFPWM is the setting resistor.

The PWM dimming frequency can be set or applied up to

30kHz with duty cycle from 0.4% to 100%.

PHASE SHIFT CONTROL

The ISL97673 is capable of delaying the phase of each

current source to minimize load transients. By default,

phase shifting is disabled as shown in Figure 20 where

the channels PWM currents are switching uniformly. The

duty cycles can be controlled by the data in PWM

Brightness Control Register via the SMBus/I2C interface,

an external PWM signal with the frequency set by the

RFPWM, or by an external PWM signal with the frequency

set by the incoming signal.

When EqualPhase = 1, the phase shift evenly spreads

the channels switching across the PWM cycle, depending

on how many channels are enabled, as shown in

Figures 21 and 22. Equal phase means there are fixed

delays between channels and such delay can be

calculated as Equations 10 and 11:

where (255/N) is rounded down to the nearest integer.

For example, if N = 6, (255/N) = 42, that leads to:

tD1 = tFPWM x 42/255

tD2 = tFPWM x 45/255

where tFPWM is the sum of tON and tOFF. N is the number

of LED channels. The ISL97673 will detect the numbers

of operating channels automatically.

ILILED ave



ILED PWM



=

(EQ. 8)

FPWM

6.66

7

10

RFPWM

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

=

(EQ. 9)

FIGURE 20. NO DELAY (DEFAULT PHASE SHIFT

DISABLED)

ILED0

ILED1

ILED2

ILED3

ILED4

ILED5

tON

tOFF

tFPWM

FIGURE 21. 6 EQUAL PHASE CHANNELS PHASE SHIFT

ILLUSTRATION

ILED0

ILED1

ILED2

ILED3

ILED4

ILED5

ILED0

tON

tOFF

PWMI

60%

40%

tFPWM

60%

40%

tD1

tD1

tD1

tD1

tD1

tD2

FIGURE 22. 4 EQUAL PHASE CHANNELS PHASE SHIFT

ILLUSTRATION

ILED1

ILED2

ILED3

ILED4

ILED1

tON

tOFF

PWMI

60%

40%

60%

40%

tD1

t

D1

t

D1

t

D2

tFPWM

(t

PWMout)

tPWMin

tD1 = Fixed Delay with Integer only while the decimal value will be discarded (eg. 63.75=63)

tD1

tFPWM

255

-------------------x 255

N

----------





=

(EQ.10)

tD2

tFPWM

255

-------------------x 255

N 1

–

 255

N

----------





–





=

(EQ.11)