4-Channel Cold-Cathode

Fluorescent Lamp Controller

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13

DPWM Soft-Start

At the beginning of each lamp burst, the DS3984 pro-

vides a soft-start that slowly increases the MOSFET

gate-driver duty cycle (see Figure 6). This minimizes

the possibility of audible transformer noise that could

result from current surges in the transformer primary.

The soft-start length is fixed at 16 lamp cycles, but the

soft-start ramp profile is programmable through the four

Soft-Start Profile registers (SSP1/2/3/4) and can be

adjusted to match the application. There are seven dif-

ferent driver duty cycles to select from to customize the

soft-start ramp (see Table 1). The available duty cycles

range from 0% to 19% in ~3% increments. In addition,

the MOSFET duty cycle from the last lamp cycle of the

previous burst can be used as part of the soft-start

ramp by using the Most Recent Value duty-cycle code.

Each programmed MOSFET gate duty cycle repeats

twice to make up the 16 soft-start lamp cycles.

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

SSP1. 0-3

LAMP CURRENT

SOFT-START PROFILE REGISTER

SOFT-START

SOFT-START (EXPANDED)

22.5Hz TO 440Hz

DPWM SIGNAL

LAMP CURRENT

LAMP CYCLE

GA_n/GB_n

MOSFET GATE DRIVERS

PROGRAMMABLE SOFT-START PROFILE WITH INCREASING MOSFET PULSE WIDTHS OVER

A 16 LAMP CYCLE PERIOD RESULTS IN A LINEAR RAMP IN LAMP CURRENT.

SSP1. 4-7

SSP2. 0-3

SSP2. 4-7

SSP3. 0-3

SSP3. 4-7

SSP4. 0-3

SSP4. 4-7

1

Figure 6. Digital PWM Dimming and Soft-Start