The MAX8790 is a high-efficiency driver for white-light-emitting
diodes (LEDs). It is designed for large liquid-crystal
displays (LCDs) that employ an array of LEDs as
the light source. A current-mode step-up controller drives
up to six parallel strings of multiple series-connected
LEDs. Each string is terminated with ballast that achieves
±1.5% current regulation accuracy, ensuring even brightness
for all LEDs. The MAX8790 has a wide input-voltage
range from 4.5V to 26V, and provides a fixed 20mA or
adjustable 15mA to 25mA full-scale LED current.
The MAX8790 has two dimming control modes to
enable a wide variety of applications. In direct DPWM
mode, the LED current is directly turned on and off by a
PWM signal. In analog dimming mode, an internal
phase-locked loop (PLL) circuit translates the PWM signal
into an analog signal and linearly controls the LED
current down to 12.5%. Below 12.5%, digital dimming is
added to allow lower average LED current down to 1%.
Both control methods provide 100:1 dimming range.
The MAX8790 has multiple features to protect the controller
from fault conditions. Separate feedback loops limit
the output voltage if one or more LEDs fail open or short.
The controller features cycle-by-cycle current limit to provide
consistent operation and soft-start capability. A thermal-
shutdown circuit provides another level of protection.
The step-up controller uses an external MOSFET, which
provides good efficiency and allows for scalable output
power and maximum operating voltage. Low feedback
voltage at each LED string (450mV) helps reduce
power loss. The MAX8790 features selectable switching
frequency (500kHz, 750kHz, or 1MHz), which allows
trade-offs between external component size and operating
efficiency.
The MAX8790 is available in a thermally enhanced,
lead-free, 20-pin, 4mm x 4mm, Thin QFN package.
Applications
