Datasheet

DS3881

Single-Channel Automotive CCFL Controller

____________________________________________________________________ 13

DPWM Soft-Start

At the beginning of each lamp burst, the DS3881 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

different driver duty cycles to select from to customize

the soft-start ramp (see Tables 4a and 4b). The avail-

able duty cycles range from 0% to 19% in ~3% incre-

ments. 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.

1615141312111098765432

SSP1. 0-3

LAMP CURRENT

SOFT-START PROFILE REGISTER

SOFT-START

SOFT-START (EXPANDED)

22.5Hz TO 440Hz

DPWM SIGNAL

LAMP CURRENT

LAMP CYCLE

GA1/GB1

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

Figure 6. Digital PWM Dimming and Soft-Start