ACT5880 datasheet(72/87 Pages) ACTIVE-SEMI

72 / 87 page

ACT5880

Rev 2, 03-Sep-13

- 72 -

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an open-LED is detected presumably. The overheat

and open-LED situation does not send out signal to

alert the system, the system could identify the

situation by ready respective bits in the ODO

Low LED bias voltage or high serial impedance may

cause false reading of an open LED situation. The

host system needs other measures to verify the true

situation and takes action accordingly.

This open LED detection function provides a way of

driving headroom detection for making adaptive

backlight driving, which is biasing the LED with a

voltage just enough for maintain the wanted current.

Typical Application Consideration

With the thoughtful function design and the

parameter design, the driver circuit in the ACT5880

provides high flexible and rich solutions for design

trade-off. The range of constant current, the range

of voltage and its programmability are enough to

most mobile backlight applications with up to 7"

display.

The thermal constraint of the ACT5880 plays the

limit of effective use of its capability. Cooperation

between the programmable step-up DC/DC of the

ACT5880 and the adaptive headroom management

could realize the feasible thermal design for high

power backlighting of up to 100mA and 40V.

The mix-modulation to power output brings another

approach of adaptive backlighting, which is to draw

the possible maximum current from a bias power

with varying voltage such as a battery by checking if

the headroom is sufficiently kept, and stabilize the

light output, what is the product of current and PWM

duty ratio, by change the PWM duty ratio.

Figure 8 shows a sample of how this approach is

used in a slightly different way, which is meaningful

sample of how the comprehensive use of the

ACT5880 capability benefits the application. In the

circuit, the step-up DC/DC output is used to bias the

WLED, and to power the USB OTG port as well.

The WLED forward voltage drop is in range of 2.8V

to 3.8V typical, dependent on LED temperature and

brightness wanted. During the backlight only

operation, the step-up DC/DC output should be

adaptively set to a voltage for maintaining proper

headroom on the driver output, in which the voltage

is normally less than 5V. During powering the OTG

port, the step-up DC/DC outputs 5V, in which case

a large drop on the LED driver is seen. For reducing

the thermal generated in the driver, the constant

current should be increased as high as possible,

and as such the drop on driver decreases, while the

PWM duty ratio is reduced for maintain the stable

brightness.

Figure 8:

A Sample Circuit for the OTG Power.

The duty ratio d for keeping the same brightness is

the function of the current i, the current at 100%

duty ratio i0, and a constant ib, as showed in the

equation below:

d=(i0-ib)/(i-ib)

where the ib is a characterized parameter for a

given LED type from the test of finding the duty ratio

dm at a current iM for keeping the same brightness,

in which the volume of iM used should be close to

the available maximum current in the particular

circuit. The curves and their linear approximations

for deriving the equation are shown in Figure 9.

Figure 9:

Deriving Adjustment for Maintaining Brightness.

ACT5880

Device

System

VBUS

3V I/O

QOTG

SW3

OUT3

ODOx

VBAT

Emission Curve

Brightness

Current

PWM

Constant

Brightness

Curve

0 brightness

approx line

(0,0)

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