The MAX1575 charge pump drives up to four white LEDs in the main display and up to two white LEDs in the sub-display with regulated constant current for uniform intensity. By utilizing adaptive 1x/1.5x charge-pump modes and very-low-dropout current regulators, it achieves high efficiency over the full 1-cell lithium-battery input voltage range. A 1MHz fixed-frequency switching allows for tiny external components, and the regulation scheme is optimized to ensure low EMI and low input ripple.
The MAX1575 uses an external resistor to set the full-scale 100% LED current. Two enable inputs, ENM and ENS, are used for simple on/off controls for the main and sub-displays, respectively. By repeatedly pulsing either enable input, the LEDs can be dimmed in multiple steps down to 5%. Once the desired brightness is set, the MAX1575 maintains that brightness setting as long as the enable input is kept high. If an enable input is kept low for more than 2ms, the LED current for the respective display is set to zero. If both enable inputs are kept low for more than 2ms, the MAX1575 enters shutdown.
The MAX1575 is available in a 16-pin 4mm x 4mm thin QFN package (0.8mm max height).