ACT5880
Rev 2, 03-Sep-13
brightness.
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
register (0x79).
Figure 8:
A Sample Circuit for the OTG Power.
VBAT
SW3
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.
ACT5880
OUT3
ODOx
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.
3V I/O
QOTG
Device
System
Typical Application Consideration
VBUS
VBUS
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 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)
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.
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.
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 9:
Deriving Adjustment for Maintaining Brightness.
Brightness
EmissionCurve
PWM
Constant
Brightness
Curve
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
.
0 brightness
approx line (0,0)
x
o
r
p
Current
p
i0
a
r
a
i
e
ib
n
i
L
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