DS3984Z datasheet(14/24 Pages) DALLAS

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4-Channel Cold-Cathode

Fluorescent Lamp Controller

____________________________________________________________________

Setting the Lamp and Dimming Clock (DPWM)

Frequencies Using External Resistors

Both the lamp and dimming clock frequencies can be

set using external resistors. The resistance required for

either frequency can be determined using the following

formula:

where K = 1600k

Ω•kHz for lamp frequency calcula-

tions. When calculating the resistor value for the dim-

ming clock frequency, K will be one of four values as

determined by the desired frequency and the POSCR0

and POSCR1 bit settings as shown in the Control

Descriptions section.

Example: Selecting the resistor values to configure a

DS3984 to have a 50kHz lamp frequency and a 160Hz

dimming clock frequency:

For this configuration, POSCR0 and POSCR1 must be

programmed to 1 and 0, respectively, to select 90Hz to

220Hz as the dimming clock frequency range. This sets

K for the dimming clock resistor (RPOSC) calculation to

Ω•kHz. For the lamp frequency resistor (RLOSC) cal-

culation, K = 1600k

Ω•kHz, which allows the lamp fre-

quency K value regardless of the frequency. The

formula above can now be used to calculate the resis-

tor values for RLOSC and RPOSC as follows:

Supply Monitoring

The DS3984 monitors both the transformer’s DC supply

and its own VCC supply to ensure that both voltage lev-

els are adequate for proper operation.

The inverter’s transformer supply (VINV) is monitored

using an external resistor-divider that is the input into a

comparator (see Figure 7) with a 2V threshold. Using

the equation below to determine the resistor values, the

supply voltage monitor (SVM) trip point (VTRIP) can be

customized to shut off the inverter when the trans-

former’s input voltage drops below any specified value.

Operating with the transformer’s supply at too low of a

level can prevent the inverter from reaching the strike

voltage and could potentially cause numerous other

problems. Proper use of the SVM can prevent these

problems. If desired, the SVM can be disabled by con-

necting the SVM pin to VCC.

The VCC monitor is used as a 5V supply undervoltage

lockout (UVLO) that prevents operation when the DS3984

does not have adequate voltage for its analog circuitry to

operate or to drive the external MOSFETs. The VCC moni-

tor features hysteresis to prevent VCC noise from causing

spurious operation when VCC is near the trip point. This

monitor cannot be disabled by any means.

Fault Monitoring

The DS3984 provides extensive fault monitoring for

each channel. It can detect open-lamp, lamp overcur-

rent, failure to strike, and overvoltage conditions. The

DS3984 can be configured to disable all channels if

one or more channels enter a Fault State, or it can be

configured to disable only the channel where the fault

occurred. Once a Fault State has been entered, the

FAULT output is asserted and the channel(s) remain

disabled until either the DS3984 is power-cycled or the

inverter’s DC supply is power-cycled. The DS3984 can

also be configured to automatically attempt to clear a

detected fault (except lamp overcurrent) by restriking the

lamp, as explained in Step 4. Configuration bits for the

fault monitoring options are located in CR1 and CR2.

TRIP









kHz

LOSC

POSC

•

1600

0 160

25 0

Ω

OSC

VINV

2.0V

SVM

EXAMPLE: R1 = 10k

Ω, R2 = 40kΩ SETS AN SVM TRIP POINT OF 10V

DS3984

Figure 7. Setting the SVM Threshold Voltage

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