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.
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.
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
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.
management LED drivers are turning LEDs, a more suitable
solution for backlighting almost any size of LCD panel, with
really conservative power consumption.
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 I2C 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.
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
FUNCTIONAL PIN DESCRIPTION
INPUT VOLTAGE SUPPLY (VIN)
IC ENABLE (EN)
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.
The active high enable terminal is internally pulled high
through pull-up resistors. Applying 0V to this terminal would
stop the IC from working.
INPUT/OUTPUT CLOCK SIGNAL (CK)
INTERNAL VOLTAGE REGULATOR 1 (VDC1)
This terminal can be used as an output clock signal
(master mode), or input clock signal (slave mode), to
synchronize more than one device.
This pin is for internal use only, and not to be used for other
purposes. A capacitor of 2.2 μF should be connected
between this pin and ground for decoupling purposes.
MASTER/SLAVE MODE SELECTION (M/~S)
INTERNAL VOLTAGE REGULATOR 2 (VDC2)
Setting this pin High puts the device into Master mode,
producing an output synchronization clock at the CK terminal.
Setting this pin low, puts the device in Slave mode, using the
CK pin as an input clock.
This pin is for internal use only, and not to be used for other
purposes. A capacitor of 2.2 μF should be connected
between this pin and ground for decoupling purposes.
INTERNAL VOLTAGE REGULATOR 3 (VDC3)
EXTERNAL PWM INPUT (PWM)
This pin is for internal use only, and not to be used for other
purposes. A capacitor of 2.2 μF should be connected
between this pin and ground for decoupling purposes.
This terminal is internally pulled down. An external PWM
signal can be applied to modulate the LED channel directly in
absence of an I2C interface.
2
BOOST COMPENSATION PIN (COMP)
CLOCK I C SIGNAL (SCK)
Passive terminal used to compensate the boost converter.
Add a capacitor and a resistor in series to GND to stabilize
the system.
Clock line for I2C communication.
2
ADDRESS I C SIGNAL (SDA)
Address line for I2C communication.
34844
Analog Integrated Circuit Device Data
Freescale Semiconductor
11
FREESCALE [ Freescale ]
