34845B datasheet(10/21 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34845

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.

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

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.

FUNCTIONAL DEVICE OPERATION

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.

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.

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.

Figures 4 and 5 illustrate the two different power up

conditions.

Figure 4. Power up sequence case 1, VIN applied

before the ENABLE and PWM signals.

No limitation for VIN ramp up time.

VIN

PWM

VOUT

Boost

Soft Star t

34845B Datasheet(PDF) 10 Page - Freescale Semiconductor, Inc

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