ISL976787IBZ-T 데이터 시트보기 (PDF) - Renesas Electronics

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ISL976787IBZ-T

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

Renesas Electronics 

ISL97687

Theory of Operation

PWM Boost Converter

The current mode PWM boost converter produces the minimal

voltage needed to enable the LED string with the highest forward

voltage drop to run at the programmed current. The ISL97687

employs current mode control boost architecture that has a fast

current sense loop and a slow voltage feedback loop. The

number of LEDs that can be driven by ISL97687 depends on the

type of LED chosen in the application. The ISL97687 is capable

of boosting up to greater than 70V and driving 4 Channels of

LEDs at a maximum of 160mA per channel.

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 ISL97687 OVP threshold is set by RUPPER and RLOWER as

shown in Equation 1:

V O U T _OVP

=

1----.-2----1------R-----U---P----P---E---R-----+-----R----L---O----W-----E---R----

RLOWER

(EQ. 1)

and VOUT can only regulate between 30% and 100% of the

VOUT_OVP such that:

Allowable VOUT = 30% to 100% of VOUT_OVP

For example, a 1MΩ RUPPER and 19kΩ RLOWER sets OVP to

65.9V. The boost can regulate down to 30% of OVP, so it can go

as low as 19.5V. If VOUT needs to be lower than this, the OVP level

must be reduced. Otherwise, VOUT will regulate to 19.5V, and the

ISL97687 may overheat. However, it’s recommended that the

OVP be set to no more than 20% above the nominal operating

voltage. This prevents the need for output capacitor voltage

ratings and the inductor current rating to be set significantly

higher than needed under normal conditions, allowing a smaller

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

placeholders.

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 operate within a range of about

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

-

RISET

+

-

PWM DIMMING

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,

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

effects 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

FN7714 Rev.3.00

Sep 13, 2017

Page 11 of 24

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