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MC34844
FUNCTIONAL DESCRIPTION
INTRODUCTION
FUNCTIONAL DESCRIPTION
INTRODUCTION
LED backlighting has become very popular for small and
medium LCDs, due to some advantages over other
backlighting schemes, such as the widely used cold cathode
fluorescent lamp (CCFL). The advantages of LED
backlighting are low cost, long life, immunity to vibration, low
operational voltage, and precise control over its intensity.
However, there is an important drawback of this method. It
requires more power than most of the other methods, and this
is a major problem if the LCD size is large enough.
To address the power consumption problem, solid state
optoelectronics technologies are evolving to create brighter
LEDs with lower power consumption. These new
technologies together with highly efficient power
management LED drivers are turning LEDs, a more suitable
solution for backlighting almost any size of LCD panel, with
really conservative power consumption.
One of the most common schemes for backlighting with
LED is the one known as “Array backlighting”. This creates a
matrix of LEDs all over the LCD surface, using defraction and
diffused layers to produce an homogenous and even light at
the LCD surface. Each row or column is formed by a number
of LEDs in series, forcing a single current to flow through all
LEDs in each string.
Using a current control driver, per row or column, helps the
system to maintain a constant current flowing through each
line, keeping a steady amount of light even with the presence
of line or load variations. They can also be use as a light
intensity control by increasing or decreasing the amount of
current flowing through each LED string.
To achieve enough voltage to drive a number of LEDs in
series, a boost converter is implemented, to produce a higher
voltage from a smaller one, which is typically used by the
logical blocks to do their function.
The 34844 implements a single channel boost converter
together with 10 input channels, for driving up to 16 LEDs per
string to create a matrix of more than 160 LEDs. Together
with its 90% efficiency and I 2 C programmable or external
current control, among other features, makes the 34844 a
perfect solution for backlighting small and medium size LCD
panels, on low power portable and high definition devices.
FUNCTIONAL PIN DESCRIPTION
INPUT VOLTAGE SUPPLY (VIN)
IC Power input supply voltage, is used internally to
produce internal voltage regulation (VDC1, VDC3) for logic
functioning, and also as an input voltage for the boost
regulator.
INTERNAL VOLTAGE REGULATOR 1 (VDC1)
This pin is for internal use only, and not to be used for other
purposes. A capacitor of 2.2 μ F should be connected
between this pin and ground for decoupling purposes.
INTERNAL VOLTAGE REGULATOR 2 (VDC2)
This pin is for internal use only, and not to be used for other
purposes. A capacitor of 2.2 μ F should be connected
between this pin and ground for decoupling purposes.
INTERNAL VOLTAGE REGULATOR 3 (VDC3)
This pin is for internal use only, and not to be used for other
purposes. A capacitor of 2.2 μ F should be connected
between this pin and ground for decoupling purposes. A
1.0 k Ω resistor between the VDC3 and VDC1 pin is
recommended for <-20 °C operation.
BOOST COMPENSATION PIN (COMP)
Passive pin used to compensate the boost converter. Add
a capacitor and a resistor in series to GND to stabilize the
system.
IC ENABLE (EN)
The active high enable pin is internally pulled high through
pull-up resistors. Applying 0 V to this pin would stop the IC
from working.
INPUT/OUTPUT CLOCK SIGNAL (CK)
This pin can be used as an output clock signal (master
mode), or input clock signal (slave mode), to synchronize
more than one device.
MASTER/SLAVE MODE SELECTION (M/~S)
Setting this pin High puts the device into Master mode,
producing an output synchronization clock at the CK pin.
Setting this pin low, puts the device in Slave mode, using the
CK pin as an input clock.
EXTERNAL PWM INPUT (PWM)
This pin is internally pulled down. An external PWM signal
can be applied to modulate the LED channel directly in
absence of an I 2 C interface.
CLOCK I 2 C SIGNAL (SCK)
Clock line for I 2 C communication.
ADDRESS I 2 C SIGNAL (SDA)
Address line for I 2 C communication.
34844
Analog Integrated Circuit Device Data
Freescale Semiconductor
14
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