ISL97635IRZ datasheet(11/28 Pages) RENESAS

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ISL97635

FN6434 Rev.3.00

Page 11 of 28

Sep 26, 2017

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 18.

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 to operate within a

range at about 500mV 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.

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.

MAXIMUM DC CURRENT SETTING

The initial brightness should be set by choosing an appropriate

value for RSET. This should be chosen to fix the maximum

possible LED current, as shown in Equation 1:

DC CURRENT ADJUSTMENT

Once RSET is fixed, the LED DC current can be adjusted through

BRTDC can be programmed from 0 to 255 in decimal and

defaults to 255 (0xFF). If left at the default value, LED current will

FIGURE 18. SIMPLIFIED CURRENT SOURCE CIRCUIT

REF

PWM DIMMING

RSET

DC DIMMING

REF

RSET

ILEDmax

733

RSET

---------------

(EQ. 1)

ILED

2.87 BRTDC RSET





(EQ. 2)

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