Design Ideas: March 16, 1995

The circuit in Fig 1 provides digital control and automatic thermal compensation for LCD contrast bias. This control and compensation eliminates the need for panel-mount hardware and frequent manual adjustment of LCD contrast in changing temperature environments. Major components in Fig 1 include an inverting current-mode PWM regulator, a programmable digital potentiometer, and a negative-temperature-coefficient (NTC) thermistor. The 5V single-supply circuit consists of all surface-mount components and occupies approximately 6.25 sq cm of board space. The circuit easily powers 240×64-dot LCDs at 80% efficiency with an adjustment range of 0 to -12V dc at 100 mA. Standby operating current is approximately 250 µA.

The circuit adjusts the output by scaling the +1.23V, V_REF output of the MAX759 PWM regulator with the divider comprised of the DS1267 digital potentiometer and the RT₁ thermistor. The DQ serial data you feed into the DS1267 with the appropriate RST and CLK timing sets the divider ratio. The MAX406 op amp scales the voltage at the wiper of the DS1267. Pin CC of the MAX759 is a virtual ground; therefore, the following loop equations determine V_OUT:

You should select the NTC thermistor so it simulates the liquid-crystal thermal behavior. The thermistor's job is to automatically adjust V_OUT in response to temperature changes. As temperature decreases, LCDs characteristically dim and require increased contrast bias to provide adequate viewing. Conversely, as temperature increases, LCDs darken and require less contrast bias. A typical 240×64 dot-reflective LCD, operating within its rated temperature range of -10 to +60°C, requires a bias operating span of -4 to -11.5V dc.

An added feature of this circuit is the shutdown function of the MAX759, which is an advantage for LCDs requiring power-supply on/off timing. Some LCD modules require sequencing of V_CC and V_BIAS when powering on/off. Neglecting the proper timing sequence results in inferior performance and causes display errors. The SHDN line is logic-level-compatible, and you should take advantage of it. Note: You can increase the regulator's output to -24V by changing the ratio of R₁ and R₂ and replacing L₁ with a simple autotransformer. (DI #1666)