Many applications are subject to stringent low power requirements. Utility meters like water and gas meters, in particular, are battery-powered and the expected battery life is up to 20 years. In-home energy displays, thermostats and load controllers run on small, low cost batteries and are always functional. These applications put severe low power design challenges on the electronics, including the LCD and the driver. New ultra low power LCD drivers are now available.
In low power applications, the display is typically the traditional Twisted Nematic (TN) reflective LCD without backlight or the transflective LCD where the backlight is switched ON with a push-button. The segmented display is typically preferred to the dot matrix display since it allows for icons rather than dots and enhances the human machine interface. The current consumption depends on the display resolution, the display area, the ‘cell gap’, i.e. the distance between the upper and the lower glass of the LCD, and the relative permittivity of the liquid crystal. Given a certain display area and resolution, the LCD manufacturer can optimize the display structure for low power applications.
The choice of the LCD drivers should not be commoditized since the LCD driver performance can be crucial for the quality, reliability and the power consumption of the overall system. The new generation of LCD drivers from NXP Semiconductors (PCF8551ATT/A, PCF8551BTT/A, PCF8553DTT/A) exceed customer expectations for high quality, reliability and very low power consumption.
Housed in the highly reliable TSSOP packages, these new LCD segment drivers require <10μA for a mid-size display and <5μA for a small-size display, even when all the display is driven ON. In addition, they provide high immunity against ESD: HBM ±5kV; CDM ± 500V. Furthermore, features like the read-back of the control registers and the display RAM data as well as software/hardware reset capabilities, guarantee secure and reliable communication with the host controller and high immunity against EMI. In fact, these drivers offer the possibility to check if a severe EMI event has occurred; if a failure is detected the host controller can easily reset the driver and re-enable the display, and thus avoid displaying false data.
The current consumption of the LCD driver strongly depends on the display load, but typically LCD driver providers specify the current consumption without any load or with the load of the tester equipment used in the production flow. Both ways, however, could lead to unrealistic data. NXP provides the graph of the current consumption as a function of the display load, which supports customers during their design phase, to properly allocate the power budgets to the different electronic functions.
The demo board OM13506 can be used to demonstrate and evaluate the PCF8553DTT/A LCD driver.
A very user-friendly GUI allows setting the control registers, switching on/off the display elements and, above all, measuring the current consumption on the pins VDD and VLCD.
Tamb=25°C; VLCD= 3.3V; VDD=3.3V;Fg=64Hz, BOOST=0.
(1) Static, all segments/elements off.
(2) Static, all segments/elements on.
(3) MUX 1:2, bias level 1/2, all segments/elements off.
(4) MUX 1:2, bias level 1/2, all segments/elements on.
(5) MUX 1:3, bias level 1/3, all segments/elements off.
(6) MUX 1:3, bias level 1/3, all segments/elements on.
(3) MUX 1:4, bias level 1/4, all segments/elements off.
(4) MUX 1:4, bias level 1/4, all segments/elements on.