DS3984Z datasheet(21/24 Pages) DALLAS

21 / 24 page

4-Channel Cold-Cathode

Fluorescent Lamp Controller

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Applications Information

Addressing Multiple DS3984s On a

Common I2C Bus

Each DS3984 responds to one of two possible slave

addresses based on the state of the address input (A0).

For information about device addressing see the I2C

Communications section.

Power-Supply Decoupling

To achieve best results, it is recommended that each

VCC pin is decoupled with a 0.01µF or a 0.1µF capacitor

to GND. Use high-quality, ceramic, surface-mount capac-

itors, and mount the capacitors as close as possible to

the VCC and GND pins to minimize trace inductance.

Setting the RMS Lamp Current

Resistor R8 in the typical operating circuit (Figure 12)

sets the lamp current. R8 = 140

Ω corresponds to a

5mARMS lamp current as long as the current waveform

is approximately sinusoidal. The formula to determine

the resistor value for a given sinusoidal lamp current is:

Component Selection

External component selection has a large impact on the

overall system performance and cost. The two most

important external components are the transformers

and n-channel MOSFETs.

The transformer should be able to operate in the 40kHz

to 80kHz frequency range of the DS3984, and the turns

ratio should be selected so the MOSFET drivers run at

28% to 35% duty cycle during steady state operation.

The transformer must be able to withstand the high

open-circuit voltage that will be used to strike the lamp.

Additionally, its primary/secondary resistance and

inductance characteristics must be considered

because they contribute significantly to determining the

efficiency and transient response of the system. Table 5

shows a transformer specification that has been utilized

for a 12V inverter supply, 438mm x 2.2mm lamp design.

The n-channel MOSFET must have a threshold voltage

that is low enough to work with logic-level signals, a low

on-resistance to maximize efficiency and limit the n-

channel MOSFET’s power dissipation, and a break-

down voltage high enough to handle the transient. The

breakdown voltage should be a minimum of 3x the

inverter voltage supply. Additionally, the total gate

charge must be less than QG, which is specified in the

Recommended DC Operating Conditions table. These

specifications are easily met by many of the dual n-

channel MOSFETs now available in SO-8 packages.

Table 6 lists suggested values for the external resistors

and capacitors used in the typical operating circuit.

ILAMP RMS

()

Figure 11. I2C Communications Examples

XXXXXXXX

A0 0

COMMUNICATIONS KEY

WRITE A SINGLE BYTE

8-BITS ADDRESS OR DATA

WHITE BOXES INDICATE THE MASTER IS

CONTROLLING SDA

NOTES

2) THE FIRST BYTE SENT AFTER A START CONDITION IS

ALWAYS THE SLAVE ADDRESS FOLLOWED BY THE

READ/WRITE BIT.

SHADED BOXES INDICATE THE SLAVE IS

CONTROLLING SDA

START

ACK

NOT

ACK

DATA

MEMORY ADDRESS

A0 0

READ A SINGLE BYTE

ASR

DATA

MEMORY ADDRESS

STOP

REPEATED

START

1) ALL BYTES ARE SENT MOST SIGNIFICANT BIT FIRST.

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