ISL97682, ISL97683, ISL97684

10

FN7689.0

March 11, 2011

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 ISL97682,

ISL97683, ISL97684 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 noticeable

visual nuisance. The number of LEDs that can be driven by the

ISL97682, ISL97683, ISL97684 depend on the type of LED

chosen in the application. The ISL97682, ISL97683, ISL97684

are capable of boosting up to 45V and typically driving 13 LEDs

in series for each of the 4 channels, enabling a total of 52 pieces

of the 3.2V/20mA type of LEDs.

OVP

The Overvoltage Protection (OVP) pin has a function of setting the

range.

The ISL97682, ISL97683, ISL97684 OVP threshold is set by

such that:

then the OVP level must be reduced and users should follow the

headroom control will be disturbed such that the channel voltage

can be much higher than expected and sometimes can prevent

the driver from operating properly.

The ratio of the OVP capacitor should be the inverse of the OVP

resistor. For example:

Enable

An EN signal is required to enable the internal regulator for normal

operation. If there is no signal for longer than 28ms, the device will

enter shutdown.

Power Sequence

There is no specific power sequence requirement for the

but not the VDC that will prevent the device from powering up.

Current Matching and Current Accuracy

Each channel of the LED current is regulated by the current

source circuit, as shown in Figure 19.

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

resistor is recommended.

Dynamic Headroom Control

The ISL97682, ISL97683, ISL97684 feature a proprietary

Dynamic Headroom Control circuit that detects the highest

forward voltage string or the lowest voltage from any of the CH

pins digitally. When the lowest CH voltage is lower than the short

signal for the boost regulator. The boost makes the output to the

correct level such that the lowest CH is at the target headroom

voltage. Since all LED stacks 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 current. The output voltage will

regulate cycle-by-cycle and it is always referenced to the highest

forward voltage string in the architecture.

Dimming Controls

The ISL97682, ISL97683, ISL97684 allow 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.

(EQ. 1)

FIGURE 19. SIMPLIFIED CURRENT SOURCE CIRCUIT

REF

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PWM DIMMING

RSET

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