FUNCTIONAL DESCRIPTION
INTRODUCTION
FUNCTIONAL DESCRIPTION
INTRODUCTION
LED backlighting has been popular for use in small LCD
displays for many years. This technology is now rapidly
replacing the incumbent Cold Cathode Fluorescent Lamp
(CCFL) in mid-size displays such as those used use in
notebooks, monitors and industrial/ consumer displays. LEDs
offer a number of advantages compared to the CCFL,
including lower power, thinner, longer lifetime, low voltage
drive, accurate wide-range dimming control and advanced
architectures for improved image quality. LEDs are also void
of hazardous materials such as mercury which is used in
CCFL.
portable equipment compared to CCFL. In large size panels,
direct backlights support advanced architectures such as
local dimming, in which power consumption and contrast ratio
are drastically improved. Edge lighting can also be used in
large displays when low cost is the driving factor.
The 34845 targets mid size panel applications in the 7” to
17” range with edge-lit backlights. The device supports LED
currents up to 30mA and supports up to 6 strings of LEDs.
This enables backlights up to 10W to be driven from a single
device. The device includes a boost converter to deliver the
required LED voltage from either a 2 or 3 cell Li-ion battery,
or a direct 12V input supply. The current drivers match the
current between devices to provide superior uniformity
across the display. The 34845 provides for a wide range of
PWM dimming from a direct PWM control input.
LED backlights use different architecture depending on the
size of the display and features required. For displays in the
7” to 17” range such as those used in notebooks, edge-lit
backlights offer very thin designs down to 2mm or less. The
efficiency of the LED backlight also extends battery life in
FUNCTIONAL DEVICE OPERATION
Figures 4 and 5 illustrate the two different power up
conditions.
POWER SUPPLY
The 34845 supports 5.0 V to 21 V at the VIN input pin. Two
internal regulators generate internal rails for internal
operation. Both rails are de-coupled using capacitors on the
VDC1 and VDC2 pins.
VIN
EN
The VIN, VDC1, and VDC2 supplies each have their own
UVLO mechanisms. When any voltage is below the UVLO
threshold, the device stops operating. All UVLO comparators
have hysteresis to ensure constant on/off cycling does not
occur.
PWM
The power up sequence for applying VIN respect to the
ENABLE and PWM signals is important since the MC34845
device will behave differently depending on how the
sequence of these signals is applied. For the case where VIN
is applied before the ENABLE and PWM signals, the device
will have no limitation in terms of how fast the VIN ramp
should be. However for the case where the PWM and
ENABLE signals are applied before VIN, the ramp up time of
VIN between 0V and 5V should be no longer than 2ms.
Boost
Soft Star t
VOUT
Figure 4. Power up sequence case 1, VIN applied
before the ENABLE and PWM signals.
No limitation for VIN ramp up time.
34845
Analog Integrated Circuit Device Data
Freescale Semiconductor
10
FREESCALE [ Freescale ]
