ISL97677IRZ【PDF 11/23 页】IC LED DVR PWM CTRL 8CH 32QFN

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ISL97677

Typical Performance Curves (Continued)

V O (1V/DIV)

V O (500mV/DIV)

I LED (20mA/DIV)

I LED (20mA/DIV)

FIGURE 21. LOAD REGULATION WITH I LED CHANGES

FROM 0% TO 100% PWM DIMMING

FIGURE 22. LOAD REGULATION WITH I LED CHANGES

FROM 100% to 0% PWM DIMMING

V O (20V/DIV)

EN (5V/DIV)

I IN (1A/DIV)

I LED (50mA/DIV)

FIGURE 23. DISABLE PROFILE

Theory of Operation

PWM Boost Converter

The current mode PWM boost converter produces the

minimal voltage needed to enable the LED string with the

highest forward voltage drop to run at the programmed

current. The ISL97677 employs current mode control

boost architecture that has a fast current sense loop and

a slow voltage feedback loop. Such architecture achieves

a fast transient response that is essential for the

notebook backlight application where the power can be

several Li-ion cell batteries or instantly change to an

AC/DC adapter without rendering a noticeable visual

nuisance. The number of LEDs that can be driven by

ISL97677 depends on the type of LED chosen in the

application. The ISL97677 is capable of boosting up to

45V and drive 8 channels of LEDs at maximum of 45mA

per channel.

11

Current Matching and Current Accuracy

Each channel of the LED current is regulated by the

current source circuit, as shown in Figure 24.

The LED peak current is set by translating the R SET

current to the output with a scaling factor of 707.9/R SET .

The source terminals of the current source MOSFETs are

designed to run at 500mV to optimize power loss vs

accuracy requirements. The sources of errors of the

channel-to-channel current matching come from the

op amps offset, internal layout, reference, and current

source resistors. These parameters are optimized for

current matching and absolute current accuracy.

However, the absolute accuracy is additionally

determined by the external R SET . A 0.1% tolerance

resistor is recommended.

FN6996.1

November 13, 2009

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