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ISL97635

SMBus 8-Channel LED Driver

Intersil 

ISL97635

Pin Descriptions (I = Input, O = Output, S = Supply) (Continued)

PIN

NAME

TYPE

DESCRIPTION

22

COMP

O

Boost compensation pin

23

VIN

S

Input voltage for the device and LED power

24

VDC

S

De-couple capacitor for internally generated supply rail. If 2.7V < VBL+ < 5.5V, apply VDC directly with

a supply voltage of 2.7V to 5.5V

Theory of Operation

PWM Boost Converter

The current mode PWM boost converter produces the

minimal voltage needed to enable the LED stack with the

highest forward voltage drop to run at the programmed

current. The ISL97635 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 a series of

drained batteries or instantly change to an AC/DC adapter

without rendering a noticable visual nuisance. The number

of LEDs that can be driven by ISL97635 depends on the

type of LED chosen in the application. The ISL97635 is

capable of boosting up to 34.5V and typically driving 9 LEDs

in series for each of the 8 channels, enabling a total of 72

pieces of the 3.5V/30mA type of LEDs.

Enable and PWMI

The EN/PWMI pin serves dual purposes; it is used as an

Enable signal and can be used as a PWM input signal for

dimming. If a PWM signal is applied to this pin, the first pulse of

minimum 40µs will be used as an Enable signal. If there is no

signal for longer than 28ms, the device will enter shutdown. The

EN/PWMI pin cannot be floating thus a 10kΩ pull-down resistor

may need to be added.

Current Matching and Current Accuracy

Each channel of the LED current is regulated by the current

source circuit, as shown in Figure 17.

The LED peak current is set by translating the RSET current

to the output with a scaling factor of 733/RSET. The source

terminals of the current source MOSFETs are designed as

100mV to minimize the power loss. The sources of errors of

the channel-to-channel current matching come from the

op amp’s offset, internal layout, reference, and current

source resistors. These parameters are optimized for current

matching and absolute current accuracy. On the other hand,

the absolute accuracy is additionally determined by the

external RSET, and therefore, additional tolerance will be

contributed by the current setting resistor. A 1% tolerance

resistor is therefore recommended.

+

-

+- REF

RSET

+

-

PWM DIMMING

DC DIMMING

FIGURE 17. SIMPLIFIED CURRENT SOURCE CIRCUIT

Dynamic Headroom Control

The ISL97635 features a proprietary Dynamic Headroom

Control circuit that detects the highest forward voltage string

or effectively the lowest voltage from any of the IIN 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 IIN pin is at the target

headroom voltage. Since all LED stacks are connected to

the same output voltage, the other IIN 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.

Dimming Controls

The ISL97635 allows two ways of controlling the LED

current, and therefore, the brightness. They are:

1. DC current adjustment

2. PWM chopping of the LED current defined in Step 1.

There are various ways to achieve DC or PWM current

control, which will be described in the following.

11

FN6434.2

December 22, 2008

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