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AN725

ADVANCED LOW POWER TECHNIQUES FOR SiM3L1XX DEVICES

Silicon Laboratories 

AN725

In constant contrast mode, the VLCD voltage is held constant at all times, regardless of the voltage on VBAT. When

VBAT is above the threshold set by the VBMTH field, the LCD charge pump regulates to the contrast level set by

the CTRST field using a variable resistor. When VBAT reaches the threshold, the charge pump enables

automatically to keep VLCD at the desired value. This mode results in the highest power consumption, but may be

required by some system and LCD architectures.

VBAT

VLCD

Figure 14. LCD0 Constant Contrast Mode

In auto-bypass contrast mode shown in Figure 15, the VLCD voltage is held constant and tracks the VBAT voltage

below the threshold. When VBAT is above the threshold voltage set by VBMTH, VLCD regulates to the

programmed contrast voltage (CTRST) using a variable resistor between VBAT and VLCD. When VBAT reaches

the monitor threshold, the controller automatically enters bypass mode and powers VLCD directly from VBAT. The

charge pump is always disabled in this mode, and the threshold and contrast levels do not have to be set to the

same value, as shown in Figure 16.

VBAT

VLCD

Figure 15. LCD0 Auto-Bypass Contrast Mode

VBAT

VLCD

CTRST

VBMTH

Figure 16. LCD0 Auto-Bypass Contrast Mode with Different Threshold and Voltage Settings

For lowest power consumption, applications should use bypass or minimum contrast modes whenever possible.

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Rev. 0.1

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