High-Efficiency Driver Handles Six Strings Of White LEDs

As a high-efficiency driver for white LEDs, the MAX8790 targets large LCDs that employ an array of LEDs as the light source. The device uses a current-mode step-up controller to drive six parallel strings of LEDs connected in multiple series, resulting in ±1.5% regulation accuracy between strings and even brightness for all LEDs. The MAX8790 has a wide input range of 4.5 to 26 V and delivers a fixed 20-mA or adjustable (15 to 25 mA) full-scale LED current.

The step-up controller uses an external MOSFET that ensures good efficiency and allows a large number of LEDs per string. Feedback voltage at each string of only 450 mV helps improve efficiency by reducing power loss. Designers can select switching frequencies of 500 kHz, 750 kHz, or 1 MHz in order to tradeoff operating efficiency against the size of external components. Two dimming control modes allow 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 phase-locked loop translates the PWM signal into an analog signal and linearly controls the LED current down to 12.5%.

The MAX8790 comes in a low-profile, thermally enhanced, 4- by 4-mm, 20-pin TQFN package. Operating range is –40°C to 85°C. Prices start at $1.95 for orders of 1000 pieces. A preassembled MAX8790 evaluation board that contains recommended external components is also available.