National Semiconductor Corp has introduced a small dual step-up DC-DC converter that drives up to five white LEDs with a constant current or an organic LED display with a constant voltage.
Well-suited for handheld devices such as flip and clam-shell cell phones and digital cameras, the LM3520 step-up converter uses pulse width modulation (PWM) mode to drive the LEDs and pulse frequency modulation (PFM) mode to drive the organic LED display. When driving the LEDs, a single external resistor is used to set the maximum LED current. The actual LED current and the brightness can be adjusted by applying a PWM signal to the enable pin. When the LED drive is disabled, the LED string is disconnected on the low side so that there is no leakage current through the LEDs.
Key features for the converter include higher than 80% peak efficiency, drives up to 5 LEDs at 20 mA/3.6V and 4 LEDs at 30 mA/3.6V for the main display, up to 20V @ 50 mA/3.6V for the sub-display, true shutdown isolation, small external components, 1MHz switching frequency, 23V of over-voltage protection, wide input voltage range of 2.7V to 5.5V, cycle-by-cycle current limit, and PWM dimming control.
National's LM3520 is available now in a small, low-profile 14-lead LLP package that measures 3 x 4 x 0.8mm and costs US$2.60 in quantities of 1,000. Lead-free package options are also available.