LCD-bias supply provides precise tracking
David Kim, Linear Technology Corp, Milpitas, CA -- EDN, May 3, 2001
Small monochrome LCD systems often require split (dual)-bias supplies with precise voltage tracking to prevent plating of the LCD. The circuit in Figure 1 provides ±18 to ±20V adjustable LCD bias voltages with 1% tracking accuracy. The circuit operates from a single 4.2 to 2.5V Li-ion cell for portable monochrome LCD applications. The circuit comprises two blocks: a negative-bias supply using the LT1611 inverting switching regulator and a positive-bias supply using the LT1636 rail-to-rail op amp. The LT1611 converts the Li-ion battery input voltage to a negative output voltage. The combination of 1.4-MHz switching frequency and a 36V internal switch results in small, low-profile circuit. LCD bias requires high voltage at low current. A charge pump consisting of C2, C4, D2, and D3 generates the negative output voltage. Some benefits of this circuit topology include zero output power during shutdown and low output ripple.
The LT1636 rail-to-rail op amp generates the positive LCD-bias output. The large capacitive-load capability, low quiescent current, and high-impedance input stage make the LT1636 suitable in this application. The LT1636 inverts the LT611's output to provide the positive LCD-bias voltage. To meet the 1% tracking requirement, you should use a precision-resistor network, such as the 664 series from BI Technologies (http://www.bitechnologies.com), for R1 and R3. The unique input stage of the LT1636 allows you to generate the VCC of the inverting op amp from the rectified switching waveform (using D1 and C1) of the LT1611 switching regulator. You adjust both LCD-bias supplies by varying the 2-kW potentiometer at the feedback node of the LT1611 regulator.
