ISL97672BIRZ データシートの表示（PDF） - Intersil

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ISL97672BIRZ

6-Channel LED Driver with Ultra Low Dimming Capability

Intersil 

ISL97672B

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 ISL97672B

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 notebook backlight applications in which drained batteries

can be instantly changed to an AC/DC adapter without

noticeable visual disturbance. The number of LEDs that can be

driven by ISL97672B depends on the type of LED chosen in the

application. The ISL97672B is capable of boosting up to 45V and

typically driving 13 LEDs in series for each of the 8 channels,

enabling a total of 104 pieces of the 3.2V/20mA type of LEDs.

Enable

The Enable pin is used to enable the device. The Enable pin should

not float. If it does, a 10k or higher pull-down resistor should be

added.

OVP and VOUT

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

overvoltage trip level as well as limiting the VOUT regulation

range.

The ISL97672B OVP threshold is set by RUPPER and RLOWER such

that:

VOUT_ovp = 1.21V * (RUPPER + RLOWER)/RLOWER

and VOUT can only regulate between 60% and 100% of the

VOUT_ovp such that:

Allowable VOUT = 60% to 100% of VOUT_ovp

if, for example, 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 VOUT is allowed to operate between 24V and 40V. If the VOUT

requirement is changed to an application of six LEDs of 21V, then

the OVP level must be reduced. Users should follow the

VOUT = (60% ~100%) OVP level requirement; otherwise, the

headroom control will be disturbed such that the channel voltage

can be much higher than expected. This can sometimes 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 19.

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 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. A 1% tolerance resistor should be used.

+

+ REF

-

-

RSET

+

-

PWM DIMMING

FIGURE 19. SIMPLIFIED CURRENT SOURCE CIRCUIT

Dynamic Headroom Control

The ISL97672B features a proprietary Dynamic Headroom

Control circuit that detects the highest forward voltage string or

effectively the lowest voltage from any of the CH0 through CH5

pins. When this lowest channel voltage is lower than the

short-circuit threshold, VSC, this voltage is used as the feedback

signal for the boost regulator. The boost adjusts 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

ensures that each channel has the same current. The output

voltage regulates cycle by cycle, and it is always referenced to the

highest forward voltage string in the architecture.

Dimming Controls

The ISL97672B 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

=

4----0----1---.-8--

RSET

(EQ. 1)

For example, if the maximum required LED current

(ILED(max)) is 20mA, rearranging Equation 1 yields Equation 2:

RSET = 401.8 ⁄ 0.02 = 20.1kΩ

(EQ. 2)

10

FN7995.0

June 13, 2012

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