ac-coupled clock
Designing a product for low-power battery operation usually means maximizing the battery life by operating the circuit on as low of a supply voltage as possible. This design used a custom LCD but, due to the number of display segments (160), a microprocessor with built-in display drivers was not feasible. Instead, the design used an MSP430F249 microprocessor and an NXP driver chip (PCF8534) that connected to the microprocessor over I2C and would operate down to 2.5 V.
To meet the operating specs, including operating from –40°C to 80°C at 2.5 V dc, the design needed an LCD using HTN fluid. Unfortunately, such displays require a bias supply voltage between 3.0 and 3.3 V. Adding a charge pump converter to generate the required bias supply voltage would have increased the design’s current consumption considerably. Increasing the circuit operating supply voltage to 3.0 V would also have increased the supply current from 200 µA to about 400 µA.
The solution was a very inexpensive (about 10 cents) and low-power circuit that generated the required bias voltage using signals already available in the design. The MSP430 allows a pin configuration that output its internal clock signals, which derive from a 32.786-kHz crystal. This signal drives a very simple charge pump.
The clock output ac-couples to a rectifier circuit referenced to the 2.5-V supply (see the figure). When the clock signal goes high, it charges the capacitor on the LCD driver’s bias input through two diodes, thereby increasing the voltage at the bias input to just over 3 V. The entire circuit, including processor and LCD driver, draws an average current of about 200 µA—half of what it would have been using an off-the-shelf charge pump IC.