Low-Cost, 6-String WLED Drivers with Quick-PWM Step-Up Converters

The MAX17129/MAX17149 are high-efficiency drivers for white LEDs. They are designed for small- to medium-sized LCDs that employ an array of LEDs as the light source. An internal switch step-up controller with Quick- PWM™ drives the LED array, which can be configured for up to 6 strings in parallel and either 11 LEDs (MAX17129) or 6 LEDs in series (MAX17149) per string. Each string is terminated with a ballast that achieves ±2% current-regulation accuracy, ensuring even LED brightness and provides an adjustable 10mA to 45mA full-scale LED current. The devices have a wide input voltage range of 6V to 26V. The MAX17129 integrates an LDO to simplify applications that have a single high-voltage supply. The devices also feature a low-input-voltage mode for applications that have a 3V to 5.5V supply voltage.

The devices support both PWM and hybrid dimming mode. In PWM dimming mode, the external PWM signal directly controls the brightness of LEDs. The dimming frequency ranges from 100Hz to 25kHz with 400ns minimum on-time. In hybrid dimming mode, the LED current amplitude can be adjusted to 25% of full-scale LED current to improve system efficiency when brightness is low.

The devices have multiple features to protect the controller from fault conditions. Separate voltage-feedback loops limit the output voltage to safe operation. The open and short-LED detection shuts down the faulty string while keeping other strings operating normally.

The devices feature cycle-by-cycle current limit on the internal switch to provide consistent operation and soft-start capability. If the devices are in current-limit condition, the step-up converter is latched off after an internal timer expires. A thermal-shutdown circuit provides another level of protection and prevents ICs from damage.

The ICs' step-up controller features an internal 0.25Ω (typ), 48V (max) power MOSFET with lossless current sense and accurate cycle-by-cycle current limit. The Quick-PWM control architecture provides fast load-transient response without requiring an external loop compensation component, simplifies the external circuitry, and saves board area. The Quick-PWM control scheme has constant off-time and adjustable pseudo-fixed frequency, which enables a wide variety of applications that can trade off component size for operating frequency. Low feedback voltage at each LED string (275mV typ at 20mA LED current) helps reduce power loss and improve efficiency.

The ICs are available in a 16-pin, thin QFN package with 0.5mm lead spacing. The package is 3mm x 3mm with a maximum thickness of 0.8mm for ultra-thin LCD panel design.