Maxim Integrated Products has introduced the MAX8790 high efficiency white light emitting diode (WLED) driver. The device is specifically designed for large-screen liquid crystal displays (LCDs) that use LED arrays as a light source. The MAX8790 uses a current-mode boost controller to drive six parallel LED strings. The built-in string current control circuit can achieve a current regulation accuracy of ±1.5% between strings to ensure uniform brightness of all LEDs. With a wide input voltage range of +4.5V to +26V, the MAX8790 provides a fixed (20mA) or adjustable (15mA to 25mA) full-scale LED current.

The boost controller uses an external MOSFET for high efficiency, adjustable output power and maximum operating voltage for driving LEDs in series with multiple lamps. The low feedback voltage (450mV) of each LED string reduces power consumption and helps increase efficiency. The selectable switching frequency (500kHz, 750kHz or 1MHz) is a compromise between external component size and operating efficiency.

The MAX8790 has two dimming modes for a variety of applications. In direct DPWM mode, the LED signal is used to directly turn on/off the LED current. In analog dimming mode, the internal phase-locked loop circuit converts the PWM signal into an analog signal that linearly controls the LED current to a minimum of 12.5%. Digital dimming adjusts the average LED current to less than 12.5% ​​and as low as 1%. Two control modes enable a wide (100:1) dimming range.

The MAX8790 has multiple features to protect the device in the event of a fault. A separate feedback loop limits the output voltage when an LED is open or shorted. If any of the feedback voltages exceeds the protection threshold, the device will shut down the controller and put the device into a latched state after the internal timer expires. The controller also features cycle-by-cycle current limiting to ensure stable operation and soft start. Thermal shutdown circuitry provides additional protection.

The MAX8790 is available in a small, thermally enhanced, 4mm x 4mm, 20-pin TQFN package and operates over the -40°C to +85°C temperature range. The device starts at $1.95 (1000 sheets, FOB USA). The assembled MAX8790 evaluation board is available, and the evaluation board contains recommended external components to help shorten the design process.

Source: Electronic Engineering