ISL97678IRZ-T 查看數據表（PDF） - Renesas Electronics

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ISL97678IRZ-T

8-Channel 45V, 50mA LED Driver

Renesas Electronics 

ISL97678

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 ISL97678

employs current mode control boost architecture, which has a

fast current sense loop and a slow voltage feedback loop. This

architecture achieves a fast transient response that is essential

for notebook backlight applications in which the power can be

several Li-ion cell batteries that instantly change to an AC/DC

adapter without rendering a noticeable visual nuisance. The

number of LEDs that can be driven by the ISL97678 depends on

the type of LED chosen in the application. The ISL97678 is

capable of boosting up to 45V and drive eight channels of LEDs

at a maximum of 45mA per channel.

Current Matching and Current Accuracy

Each channel of the LED current is regulated by the current

source circuit, as shown in Figure 26.

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

output with a scaling factor of 707.9/RSET. The sink terminals of

the current source MOSFETs are designed to operate within a

range at about 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.

However, the absolute accuracy is additionally determined by the

external RSET. A 0.1% tolerance resistor is recommended.

.

+-

+- REF

RSET

+-

PWM DIMMING

FIGURE 26. SIMPLIFIED CURRENT SOURCE CIRCUIT

Dynamic Headroom Control

The ISL97678 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. 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 CH pin is at the target headroom voltage.

Because 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 ensures that

each channel has the same programmed current. The output

voltage regulates cycle-by-cycle and is always referenced to the

highest forward voltage string in the architecture.

OVP and VOUT Requirement

The Overvoltage Protection (OVP) pin sets the overvoltage trip

level and limits the VOUT regulation range.

The ISL97678 OVP threshold is set by RUPPER and RLOWER as

shown in Equation 1:

VOUT_OVP = 1.21V  RUPPER + RLOWER  RLOWER

(EQ. 1)

VOUT can regulate only 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 RUPPER and RLOWER 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.

Dimming Controls

The ISL97678 provides 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.

There are various ways to achieve DC or PWM current control,

which will be described in the following.

In any dimming controls, the EN pin must be high. EN is a high

voltage pin that can be applied with a digital signal or tied

directly to VIN for enable function.

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 = -R7---0-S--7--E--.-9-T-

(EQ. 2)

Alternatively, the RSET can be replaced by a digital potentiometer

for adjustable current.

FN6998 Rev.3.00

Sep 8, 2017

Page 11 of 17

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