ISL976787IBZ-T 데이터 시트보기 (PDF) - Renesas Electronics

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ISL976787IBZ-T

4-Channel LED Driver with Phase Shift Control and 10-Bit Dimming Resolution

Renesas Electronics 

ISL97687

PWM Dimming Frequency Adjustment

The dimming frequencies of serial interface and ACTL modes are

set by an external resistor at the PWM_SET pin, as shown in

Equation 3:

fPWM

=

---1---.--6---6----5----------1---0---7-

RPWMSET

(EQ. 3)

where fPWM is the desirable PWM dimming frequency and

RPWMSET is the setting resistor.

VSYNC FUNCTION

The VSYNC function is used to provide accurate LED dimming

frequencies and make sure that the video data is properly

aligned with the frame rate. A phase locked loop (PLL) is used to

lock the frequency to a multiple of the frame rate. Additionally,

the phase of the PWM output is aligned with the frame rate to

provide very predictable video performance. In VSYNC mode, the

PWM_SET/PLL pin is used as the PLL loop compensation pin and

needs a loop filter connected between it and ground.

Frame rates between 30Hz and 300Hz are supported, and an

automatic frequency detection circuit will provide the same

output frequency at 30, 60, 120, 180, 240, and 300Hz.

Additionally, the PWM dimming frequency can be pre-selected to

any of the following values shown in Table 1 (Note that for the

60Hz range, the frequencies will be scaled by a factor of

framerate/60Hz and for the 120Hz range they will be scaled by a

factor of framerate/120Hz).

TABLE 1. PRE-SELECTED PWM DIMMING FREQUENCY AT VSYNC MODE

DIMMING FREQUENCY

(Hz)

(kHz)

(kHz)

(kHz)

180

1.26

5.70

13.38

240

1.38

6.18

13.86

300

1.50

6.66

14.34

360

1.62

7.14

14.82

420

1.74

7.62

15.30

480

1.86

8.10

15.78

540

1.98

8.58

16.26

600

2.10

9.06

16.74

660

2.34

9.54

17.22

720

2.58

10.02

17.70

780

2.88

10.50

18.18

840

3.36

10.98

18.66

900

3.78

11.46

19.14

960

4.20

11.94

19.62

1.02k

4.74

12.42

20.10

1.14k

5.22

12.90

20.58

FN7714 Rev.3.00

Sep 13, 2017

Phase Shift Control

The ISL97687 is capable of delaying the phase of each current

source within the PWM cycle. Conventional LED drivers present

the worst load transients to the boost converter, by turning on all

channels simultaneously, as shown in Figure 23. The ISL97687

can be configured to phase shift each channel by 90°,

individually turning them on and off at different points during the

PWM dimming period, as shown in Figure 24. At duty cycles

below 100%, the load presented to the boost will peak at a lower

level and/or spend less time at the peak, when compared to that

of a conventional LED driver, as shown in Figure 23. Additionally,

load steps are limited to the LED current of one CH pin, one

quarter of that of a standard driver. This can help reduce

transients on VOUT and also reduces audio noise by limiting the

magnitude of changes in magnetic field required in the inductor

needed to track the load. Audio noise is also generally improved

for PWM frequencies in the audio band, as the effective

frequency of the boost load is multiplied by a factor of 4,

meaning that, for example, a 5kHz LED frequency offers an

effective boost load frequency of 20kHz.

ICH4

ICH3

ICH2

ICH1

ICH_TOTAL

TIME

FIGURE 23. NON PHASE SHIFT PWM DIMMING AT 50% DUTY CYCLE

ICH4

ICH3

ICH2

ICH1

ICH_TOTAL

TIME

FIGURE 24. PHASE SHIFT PWM DIMMING AT 50% DUTY CYCLE

VOUT Control when LEDs are Off

When the backlight is enabled but all LEDs are off (i.e., during

the PWM off times), the switching regulator of a typical LED

drivers will stop switching, which can allow the output to begin to

discharge.

Page 13 of 24

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