ISL976787IBZ-T Просмотр технического описания (PDF) - Intersil

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ISL976787IBZ-T

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

Intersil 

ISL97687

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

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.

This is not a problem when the LED off times are short and the

duty cycle is running at a high duty cycle, or the output

capacitance is large. However, it presents two problems. First, for

low duty cycles at low frequencies, VOUT can droop between

on-times, resulting in under-regulation of the current when the

LEDs are next switched on. Second, at high PWM frequencies or

very low duty cycles, LED on-times can be shorter than the

minimum number of boost cycles needed to ramp up the

inductor current to the required level to support the load. For

example, a 1% on-time while running at 20kHz PWM dimming

frequency is only 500ns. If the boost switching frequency is set at

500kHz, this only represents a quarter of a switching cycle per

LED on-time, which may not be sufficient to ramp the inductor

current to the required level.

The ISL97687 incorporates an additional PFM switching

mechanism that allows the boost stage to continue to switch at

low current levels in order to replace the energy lost from the

output capacitor due to the OVP stack resistance and capacitor

self discharge. For very short pulses, this also means that the

charge delivered to the LEDs in the on-times is provided entirely

13

FN7714.0

September 15, 2011

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