ISL97678IRZ-T 查看數據表（PDF） - Intersil

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ISL97678IRZ-T

8-Channel 45V 50mA LED Driver

Intersil 

ISL97678

Typical Performance Curves (Continued)

VO (20V/DIV)

EN (5V/DIV)

IIN (1A/DIV)

ILED (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 ISL97678 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

ISL97678 depends on the type of LED chosen in the

application. The ISL97678 is capable of boosting up to

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

per channel.

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 RSET

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

The source terminals of the current source MOSFETs are

designed to run at 500mV to optimize power loss versus

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 RSET. A 0.1% tolerance

resistor is recommended.

.

+

-

+- REF

RSET

+

-

PWM DIMMING

FIGURE 24. SIMPLIFIED CURRENT SOURCE CIRCUIT

Dynamic Headroom Control

The ISL97678 features a proprietary Dynamic Headroom

Control circuit that detects the highest forward voltage

string or effectively the lowest voltage from any of the

CH pins. When this lowest IIN voltage is lower than the

short circuit threshold, VSC, such voltage will be used as

the feedback signal for the boost regulator. The boost

makes the output to the correct level such that the

lowest CH pin is at the target headroom voltage. Since all

LED strings are connected to the same output voltage,

the other CH pins will have a higher voltage, but the

regulated current source circuit on each channel will

ensure that each channel has the same programmed

current. The output voltage will regulate cycle-by-cycle

and is always referenced to the highest forward voltage

string in the architecture.

11

FN6998.1

November 5, 2009

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