Using pulse width modulation scheme for LCD back lighting an app note from Hantronix. Link here (PDF)
LED back lights on LCD modules are generally driven with a dc voltage through a current limiting resistor. This simple approach is perfectly acceptable for most applications. When the primary consideration is an extra bright display, the lowest possible power consumption, or a back light that can be controlled over a very wide brightness range another method is needed. The purpose of the paper is to describe this method.
For over the normal range temperature, Hantronix presents a simple temperature compensation circuit to correct LCD contrast, Link here (PDF)
The optimal contrast setting for LCD displays varies with ambient temperature. For most applications this variation in contrast is tolerable over the “normal” temperature range of 0°C to +50°C. Most Hantronix LCD modules are available with an extended temperature range option which allows the display to operate from -20°C to +70°C. The changes in contrast are NOT usually tolerable over this wide a range of temperatures, which means a way of adjusting the contrast voltage as the ambient temperature changes must be provided.
As the temperature decreases the LCD fluid requires a higher operating voltage in order to maintain a given optical contrast. One way to provide for this is to give the user control of the contrast. This is a simple solution but quite often its not desirable or practical.
Another application note from Hantronix, Inc. on simple to digitally controlled efficient power supply for LCD display contrast. Link here (PDF)
Many LCD display modules require a negative or positive voltage that is higher than the logic voltage used to power an LCD. This voltage, called Vl, Vee or the bias voltage, would require a second power supply in the application device. If this power source is not available the LCD bias voltage must be generated from an existing voltage, either the logic voltage (+3.0-+5v) or a battery. This application note describes circuits for generating either a negative or positive LCD bias voltage from such a voltage source.
The LCD bias voltage is used to power the circuits that drive the LCD glass. This voltage sets the contrast level of the LCD. Since any changes in this voltage will cause a visible change in the contrast of the LCD it must be regulated to better than about 200mV. Any noise or ripple on this signal may cause visible artifacts on the LCD so they must be kept below about 100mV.
An application note from Hantronix, Inc. on LCD viewing angles and how it influences the selection of the right LCD for your application. Link here (PDF)
LCD displays have a limited viewing angle. They lose contrast and become hard to read at some viewing angles and they have more contrast and are easier to read at others. The size of the viewing angle is determined by several factors, primarily the type of LCD fluid and the duty cycle. Because the viewing angle tends to be smaller than most people would like, a bias is designed into the module at the time it is manufactured. This means the nominal viewing angle is offset from the perpendicular by some amount. Several versions of the LCD module are then offered with this bias set to different angles or positions to accommodate as many applications as possible. The term “bias angle” is often used erroneously with the term “viewing angle”.