Dallas—Dallas Semiconductor touts their
highly integrated DS3992 two-channel, and DS3994 four-channel controllers for cold-cathode fluorescent lamps (CCFLs) as low-cost inverter solutions for liquid crystal display (LCD) backlighting applications.
The DS3992 supports a one-lamp-per-channel configuration with fully independent lamp control, claiming superior lamp current balance compared to single-channel solutions. The DS3994, with a light-staggering feature that claims improved LCD video quality for larger arrays, supports configurations of one to four channels and allows multiple DS3994 controllers to be cascaded.
The DS3992 and DS3994 use a push-pull drive scheme to convert a DC voltage (5 to 24 volts) to a 600 to 1200 VRMS, and 300 to 1400 VRMS near-sinusoidal AC output, respectively, for powering the CCFLs. The push-pull drive scheme uses a minimal number of external components.
Each DS3992/DS3994 channel drives two logic-level n-channel MOSFETs that are tied between the ends of a step-up transformer and ground. The transformer has a center tap on the primary side that is connected to the DC supply. The DS3992/DS3994 alternately turn on the
two MOSFETs to create the high-voltage AC waveform on the secondary side. Using a pulse-duration scheme for controlling the MOSFET turn-on time, the
controllers accurately control the current flowing through a CCFL.
A series resistor on the low-voltage side of the CCFL facilitates current monitoring. The voltage developed across this resistor is fed to the lamp current monitor (LCM) input on each device. The DS3992/DS3994 compare the voltage across the resistor against an internal reference voltage to determine the duty cycle for the MOSFET gates.
Click here to access the DS3992 datasheet, and here for the DS3994's. The DS3992 is available in a
16-pin SO package, and the DS3994 comes in a 28-pin SO package. Pricing starts at 60 cents each for the DS3992 and 85 cents each for the DS3994 (1000-up, FOB USA).
Maxim Integrated Products, 1-800-998-8000, www.maxim-ic.com