powerZONE Products for the week of September 22, 2003
Microsemi Corporation Says . . .
LX1745: ICs Generate Boost Drive for LEDs and Adjustable
Bias Voltages for LCD Displays
Dual Function in Single Surface Mount Packages Save Space,
Cut System Cost
Greater Than 90% Efficiency: Ideal for Portable Applications
Microsemi Corporation, a leading manufacturer of analog and mixed signal integrated circuits, has introduced two new Ics that in a single compact package boost a low voltage supply to drive a series string of white or color LEDs, while simultaneously providing an adjustable bias voltage for the LCD displays they light.
The LX1744 PFM boost controller provides an adjustable LED drive and one adjustable +/- bias supply output. The LX1745 controller includes the LED drive and two adjustable bias supplies. With efficiency rated at greater than 90%, the new regulators save space and system cost in handheld applications that include PDAs, smart-phones, digital cameras and pagers.
The two boost controllers are optimized for portable power management. They include a logic-controlled shutdown current of less than 1microamp, with a choice of input voltage ranging from 1.6V to 6.0V. Start-up is guaranteed at 1.6V, with sustained operation from as little as 1.1V.
LED drive currents are easily programmed using one external current sense
resistor in series with the LEDs. Depending upon the MOSFET selected, the
LX1744 can support an LED drive in excess of 1.5 watts and the LX1745 in
excess of 1.0 watt.
analogZONE Says...
Of these two parts the 1-W LX1745 will be the most popular because of the second LCD bias supply it generates, and it will find design sockets particularly in smart phones and handheld computers.
The part is a triple output PFM boost converter for LCD biasing and LED drive. Converter switching for the LCD biasing outputs is enabled when the voltage on one of the two feedback pins falls below the on-chip reference. This pulls the gate of one of the on-chip MOSFETs high and connects one of the two external boost inductors to ground causing it to build up energy storage. This happens until the internal reference threshold is reached and the energy is then transferred to the load. After a 300 ns fixed off-time the cycle will be repeated until the feedback reaches the reference voltage level. The same system is used for the LED drive but there the MOSFET is external and the threshold is set by an external resistor on the CS pin.
A voltage divider resistor sets the feedback voltage to produce LCD bias outputs between the rail voltage and 25 V, but the voltage can be dynamically varied for contrast control with a voltage on the ADJ pin, over-riding the internal reference. The signal can be a PWM signal (>100 kHz) or a dc bias. A negative LCD bias can also be derived using an external inductorless charge pump. The voltage of the negative bias mirrors the positive voltage produced, but is unregulated -- but that can be achieved by adding a standard bipolar pass element.
The current drive for the LED chain is also set with a single external resistor and brightness can be controlled using a voltage (or PWM signal) applied to the BRT pin.
The part can be operated from 1.6 V to 6.0 V and the maximum typical quiescent with a 3-V rail is 140 µA. Efficiencies for the LED driver output reach 90% for loads over 5 mA, while the peak LCD bias efficiencies reach about 85% at 10 mA load and fall either side of that current.
The LX1745 is in production in a TSSOP-20, priced at $1.94 in 10-k piece lots.