EYE on NPI: Richtek RT4539 36 V I²C Controlled 6-Channel WLED Driver #EYEonNPI #Digikey @DigiKey
35This week's EYE ON NPI is a bright-eye'd and bushy-tailed go-getter, who can juggle multiple tasks at once: it's the Richtek RT4539 36 V I²C Controlled 6-Channel WLED Driver (https://www.digikey.com/en/product-highlight/r/richtek/rt4539-36-v-6-ch-wled-driver)! This chip can do it all: it's a boost converter, it's a calibrated constant-current sink, it's a PWM controller, it's a gamma-corrector, and it's got I2C for controlling all the settings. It's designed for use in laptop or tablet backlight control, and all the little extras that were designed in make it excellent for other sorts of lighting drives - just cause it says its for white LEDs doesn't mean you have to only use it for that purpose!
The first feature of this chip is the boost converter. Since most LED backlights (and even some large LED clusters) have the LEDs in series to keep them current balanced, we often need to boost the battery voltage up to 20V+, this chip can go up to 36V output from 2.7 to 24V input - which means it can handle up to 10 LEDs at 3.3V forward drops, and still have a little headroom. The boost FET switch is internal so only an inductor and diode are required. Since this is a constant-current chip, the output voltage will fluctuate as necessary to support the current draw needed. The booster frequency can be adjusted over I2C to balance flickering/beat frequencies and efficiency vs low power usage.
The second feature is the constant current string driver. As mentioned, large backlights try to string LEDs together to balance the current, but once you get past about 3-5" diagonal, there's usually more than one string. So now you want to make sure each string is balanced. Inside are comparator/differential drivers that use matched transistors and resistors with current mirroring setup to make sure you get the same exact current through each string.
The third feature of the RT4539 is the PWM driver (https://en.wikipedia.org/wiki/Pulse-width_modulation). Yes you can reduce the current through the strings but its easier to get a wide range of brightness values by keeping a constant current and then PWMing the output sink drivers. It's normal to see linear PWM drivers: where you can go from 0-255, say for 8-bit control and that translates to a linear duty cycle. This chip goes above-and-beyond and provides an exponential "gamma corrected" (https://learn.adafruit.com/led-tricks-gamma-correction) output control curve if desired. This gives a more natural and useful brightness curve where there's more granularity at low brightness values, often used for night modes.
The fourth (and final?) feature to highlight is that all of the settings for the capabilities are available over I2C! From the booster frequency to sink current, to the PWM style and duty cycles to extras like smooth dimming transition features, there's a register map for easy configuration and adjustment, making his chip trivial to integrate into any design with a microcontroller or microcomputer.
The Richtek RT4539 (https://www.digikey.com/short/th42j03z) is super-duper new, so it's not yet available for purchase on Digi-Key's website - keep an eye out for when this chip becomes available or contact your Digi-Key sales rep and they'll be able to wrangle you some samples!