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LED driver with current balancing suits LCD apps

Posted: 15 Mar 2007 ?? ?Print Version ?Bookmark and Share

Keywords:LED? white LED? MOSFET? Maxim? controller?

Maxim Integrated Products has introduced the MAX8790, a high-efficiency driver for white LEDs designed for large LCD panels. This device uses a current-mode step-up controller to drive six parallel strings of LEDs connected in multiple series. This built-in string-current-control circuit achieves 1.5 percent regulation accuracy between strings, which ensures even brightness for all LEDs. The MAX8790 has a wide +4.5V to +26V input-voltage range and provides a fixed (20mA) or adjustable (15mA to 25mA) full-scale LED current.

Improved efficiency
The step-up controller uses an external MOSFET for good efficiency, enables scalable output power and maximum operating voltage, and allows a large number of LEDs per string. Low feedback voltage at each LED string (450mV) helps improve efficiency by reducing power loss. Selectable switching frequency (500kHz, 750kHz or 1MHz) allows tradeoffs between external component size and operating efficiency.

The MAX8790 has two dimming control modes to enable a wide variety of applications. In direct DPWM mode, LED current is directly turned on and off by a PWM signal. In analog dimming mode, an internal PLL circuit translates the PWM signal into an analog signal and linearly controls the LED current down to 12.5 percent. Digital dimming enables average LED current below 12.5 percent and down to 1 percent. Both control methods provide a wide (100:1) dimming range.

Fault protection
The MAX8790 has multiple features to protect the controller from fault conditions. Separate feedback loops limit the output voltage if one or more LEDs fall open or short. If one of the feedback voltages exceeds the protection threshold, the controller shuts down and latches off after an internal timer expires. The controller features cycle-by-cycle current limit, which provides consistent operation and soft-start capability. A thermal-shutdown circuit provides an additional level of protection.

The MAX8790 is available in a low-profile, thermally enhanced, 4-by-4mm, 20-pin TQFN package and operates over the -40C to 85C temperature range. Prices start at $1.95 (1,000-up, FOB USA). A preassembled MAX8790 evaluation kit that contains recommended external components is available to reduce design time.

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