ISL97636 Просмотр технического описания (PDF) - Renesas Electronics

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ISL97636

8-Channel LED Driver

Renesas Electronics 

ISL97636

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

ISL97636 employes 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 changed

to an AC/DC adapter without rendering a noticeable visual

nuisance. The number of LEDs that can be driven by ISL97636

depends on the type of LED chosen in the application. The

ISL97636 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 for 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 10k 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 and reference and these

parameters are optimized for current matching and absolute

current accuracy. The absolute accuracy is also determined by

the external RSET, and therefore a 1% tolerance resistor should

be used.

+-

+- REF

RSET

+-

PWM DIMMING

FIGURE 17. SIMPLIFIED CURRENT SOURCE CIRCUIT

Dynamic Headroom Control

The ISL97636 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 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 ISL97636 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.

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:

ILEDmax = R---7--S--3--E-3---T-

(EQ. 1)

DC CURRENT ADJUSTMENT

RSET can be a DCP (Digitally Controlled Potentiometer) for DC

current adjustment but minimum resistance should not be

lower than 21k for a maximum of 35mA.

FN6570 Rev 0.00

May 9, 2008

Page 10 of 18

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