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ISL97671A

6-Channel SMBus/I2C or PWM Dimming LED Driver with Phase Shift Control

Intersil 

ISL97671A

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 ISL97671A

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 noticeably visual nuisance. The

number of LEDs that can be driven by ISL97671A depends on the

type of LED chosen in the application. The ISL97671A is capable

of boosting up to 45V and drive 6 channels of LEDs.

Enable

The EN pin is used to enable or disable the ISL97671A operation. It

is a high voltage pin that can be tied directly to VIN up to 26.5V if the

system lacks of I/O for enable signal.

OVP and VOUT Requirement

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

overvoltage trip level as well as limiting the VOUT regulation

range.

The ISL97671A OVP threshold is set by RUPPER and RLOWER as

shown in Equation 1:

VOUT_OVP = 1.21V × (RUPPER + RLOWER) ⁄ RLOWER

(EQ. 1)

VOUT can only regulate between 64% and 100% of the VOUT_OVP

such that:

Allowable VOUT = 64% to 100% of VOUT_OVP

For example, if 10 LEDs are used with the worst case VOUT of 35V.

If R1 and R2 are chosen such that the OVP level is set at 40V,

then the VOUT is allowed to operate between 25.6V and 40V. If

the requirement is changed to a 6 LEDs 21V VOUT application,

then the OVP level must be reduced and users should follow

VOUT = (64% ~100%) OVP requirement. Otherwise, the headroom

control will be disturbed such that the channel voltage can be

much higher than expected and sometimes it can prevent the

driver from operating properly.

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

resistors. For example, if RUPPER/RLOWER = 33/1, then

CUPPER/CLOWER = 1/33 with CUPPER = 100pF and CLOWER = 3.3nF.

Current Matching and Current Accuracy

Each channel of the LED current is regulated by the current

source circuit, as shown in Figure 20.

The LED peak current is set by translating the RSET current to the

output with a scaling factor of 401.8/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. 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 20. SIMPLIFIED CURRENT SOURCE CIRCUIT

Dynamic Headroom Control

The ISL97671A features a proprietary Dynamic Headroom

Control circuit that detects the highest forward voltage string or

effectively the lowest voltage from any of the CH0-CH5 pins.

When this lowest channel 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 channel pin is at the target headroom

voltage. Since all LED stacks are connected to the same output

voltage, the other channel 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 ISL97671A provides SMBus/I2C controlled PWM or DC

dimming where the users need to turn the LEDs on through the

SMBus/I2C communications (see the “SMBus/I2C

Communications” on page 18). The ISL97671A also provides

PWM dimming by external PWM signal where the SMBCLK and

SMBDAT pins are grounded or pulled low and the dimming

frequency can be adjusted. The ISL97671A also allows Direct

PWM Dimming where the output duty cycle and dimming

frequency follow the input PWM signal. The three dimming mode

selection is summarized in Table 1.

TABLE 1. DIMMING MODE SELECTION

SMBCLK/SCL Pin

SIGNAL

SMBDAT/SDA

PIN SIGNAL

FPWM PIN

DIMMING MODE

SELECTION

Low

Low

Resistor to PWM Dimming with

ground Adjustable Dimming

Frequency, Phase Shift

Enabled

Low

Low

Tie to VDC Direct PWM Dimming

SMBus clock

I2C clock

SMBus data

I2C data

Resistor to SMBus Controlled Dimming

ground

Resistor to I2C Controlled Dimming

ground

11

FN7709.1

March 24, 2011

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