DS3984Z datasheet(9/24 Pages) DALLAS

9 / 24 page

4-Channel Cold-Cathode

Fluorescent Lamp Controller

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DS3984

OVERVOLTAGE

LAMP OVERCURRENT

LSE BIT AT CR1.0

LAMP REGULATION

CHANNEL ENABLE

CHANNEL FAULT

DIGITAL

CCFL

CONTROLLER

LAMP OUT

1 OF 4 CHANNELS

400mV

LCM

LAMP CURRENT

MONITOR

1.0V

2.0V

1.0V

256 LAMP CYCLE

INTEGRATOR

PEAK

DETECT/

HOLD

PEAK

DETECT

OVD

OVERVOLTAGE

DETECTION

MOSFET

GATE

DRIVERS

GATE

DRIVERS

256 LAMP CYCLE

INTEGRATOR

DPWM SIGNAL

512 x LAMP FREQUENCY

[20.48MHz TO 40.96MHz]

PHASED LAMP FREQUENCY

[40kHz TO 80kHz]

Figure 1. Per Channel Logic Diagram

Detailed Description

The DS3984 uses a push-pull drive scheme to convert

a DC voltage (5V to 24V) to the high-voltage (600VRMS

to 1200VRMS) AC waveform that is required to power

the CCFLs. The push-pull drive scheme uses a minimal

number of external components, which reduces

assembly cost and makes the printed circuit board (PC

board) design easy to implement. The push-pull drive

scheme also provides an efficient DC-to-AC conversion

and produces near-sinusoidal waveforms.

Each DS3984 channel drives two logic-level n-channel

MOSFETs that are connected between the ends of a

step-up transformer and ground (see Figure 1 and the

Typical Operating Circuit). The transformer has a center

tap on the primary side that is connected to a DC voltage

supply. The DS3984 alternately turns on the two

MOSFETs to create the high-voltage AC waveform on the

secondary side. By varying the duration of the MOSFET

turn-on times, the controller is able to accurately control

the amount of current flowing through the CCFL.

A resistor in series with the CCFL’s ground connection

enables current monitoring. The voltage across this

resistor is fed to the lamp current monitor (LCM) input

on the DS3984. The DS3984 compares the peak resistor

voltage against an internal reference voltage to deter-

mine the duty cycle for the MOSFET gates. Each CCFL

receives independent current monitoring and control,

which results in equal brightness across all of the lamps

and maximizes the lamp’s brightness and lifetime.

EEPROM Registers and I2C-Compatible

Serial Interface

The DS3984 uses an I2C-compatible serial interface for

communication with the on-board EEPROM configura-

tion registers and user memory. The configuration regis-

ters—four Soft-Start Profile registers (SSP1/2/3/4) and

two Control Registers (CR1/2)—allow the user to cus-

tomize many DS3984 parameters such as the soft-start

ramp rate, the lamp and dimming frequency sources,

fault-monitoring options, and channel enabling/disabling.

The eight bytes of nonvolatile user memory can be used

to store manufacturing data such as date codes, serial

numbers, or product identification numbers.

The device is shipped from the factory with the con-

figuration registers programmed to a set of default

configuration parameters. To inquire about custom

factory programming, please send an email to

MixedSignal.Apps@dalsemi.com.

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