ISL976787IBZ-T 查看數據表（PDF） - Intersil

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ISL976787IBZ-T

4-Channel LED Driver with Phase Shift Control and 10-Bit Dimming Resolution

Intersil 

ISL97687

and cheaper solution, as well as keeping the maximum voltages

and currents that can be seen in the system during fault

conditions at less extreme levels.

Parallel capacitors should be placed across the OVP resistors

such that RUPPER /RLOWER = CLOWER /CUPPER . Using a CUPPER

value of at least 30pF is recommended. These capacitors reduce

the AC impedance of the OVP node, which is important when

using high value resistors. The ratio of the OVP capacitors should

be the inverse of the OVP resistors. For example, if

RUPPER/RLOWER = 33/1, then CUPPER /C LOWER = 1/33 with

CUPPER = 100pF and CLOWER = 3.3nF. These components are not

always needed, but it is highly recommended to include

replacements to populate them if necessary.

Current Matching and Current Accuracy

The LED current in each channel is regulated using an active

current source circuit, as shown in Figure 20. The peak LED

current is set by translating the RISET current to the output with a

scaling factor of 2919/RISET. The drain terminals of the current

source MOSFETs are designed to run at 750mV to optimize

power loss versus accuracy requirements. The sources of

channel-to-channel current matching error come from the

op amp offsets, reference voltage, and current source sense

resistors. These parameters are optimized for current matching

and absolute current accuracy. However, the absolute accuracy is

additionally determined by the external RISET. A 0.1% tolerance

resistor is therefore recommended.

+

-

+- REF

RSET

+

-

PWM DIMMING

and is updated as needed, to allow for temperature and aging

affects in the LEDs.

Dimming Controls

The ISL97687 provides two basic ways to control the LED current,

and therefore, the brightness. These are described in detail in

subsequent sub-sections, but can be broadly divided into the

following two types of dimming:

Step 1. LED DC current adjustment

Step 2. PWM chopping of the LED current defined in Step 1.

LED DC Current Setting

The initial brightness should be set by choosing an appropriate

value for the resistor on the ISET1/2 pins. This resistor must

connect to AGND, and should be chosen to fix the maximum

possible LED current:

ILEDmax

=

-2----9---1----9--

RISET

(EQ. 2)

The ISL97687 includes two built-in levels of current, individually

set by the resistors on ISET1 and ISET2, according to Equation 2,

which can be switched between by using the CSEL pin.

CSEL = 0: The current setting is based on ISET1

CSEL = 1: The current setting is based on ISET2

This is typically used in 3D systems to provide a higher current

level in 3D modes, but is not restricted to this application. CSEL

can be switched in operation and updates immediately in direct

PWM mode, and at the start of the next PWM dimming cycle in

other modes.

LED DC DIMMING

It is possible to control the LED current by applying a DC voltage

VDIM to the ISET1/2 pin via a resistor as in Figure 21.

ISET

VISET: 1.21V

RISET

RDIM

VDIM

FIGURE 20. SIMPLIFIED CURRENT SOURCE CIRCUIT

Dynamic Headroom Control

The ISL97687 features a proprietary dynamic headroom control

circuit that detects the highest forward voltage string, or

effectively the lowest voltage from any of the CH pins. The

system will regulate the output voltage to the correct level to

allow the channel with the lowest voltage to have just sufficient

headroom to correctly regulate the LED current. Since all LED

strings 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

correct current level. The output voltage regulation is dynamic,

FIGURE 21. LED CURRENT CONTROL WITH VDIM

If the VDIM is above VISET 1.21V, the brightness will reduce, and

vice versa. In this configuration, it is important that the control

voltage be set to the maximum brightness (minimum voltage)

level when the ISL97687 is enabled, even if the LEDs are not lit

at this point. This is necessary to allow the chip to calibrate to the

maximum current level that will need to be supported.

Otherwise, on-chip power dissipation will be higher at current

levels above the start-up level. Dimming with this technique

should be limited to a minimum of 10~20% brightness, as LED

current accuracy is increasingly degraded at lower levels.

11

FN7714.0

September 15, 2011

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