Generating the negative rail in LCDs
Liquid crystal displays (LCDs) are frequently used in portable devices that run off of one- or two- cell lithium batteries. LCDs are chosen for these applications due to their small size and weight, as well as efficiency. To prolong the portable device’s battery life, the display and its power converters must work together to be efficient at converting the limited battery energy into the proper light output.
Oftentimes, the LCD’s negative supply voltage is the most difficult to produce because there are many less options available in the market for producing negative voltages. When options are available, they frequently are not optimized for the portable market’s needs of small size, low component count, efficiency, and design simplicity.
This article compares three different LCD power converter solutions on the basis of these parameters, while reviewing the basics of inverting buck-boost converters, which produce an LCD’s negative voltage rail.
Inverting buck-boost converter basics
A buck converter takes a higher voltage but produces a lower voltage. The boost converter takes a lower voltage but produces a higher voltage. Finally, the inverting buck-boost converter takes a positive voltage and creates a negative voltage. This voltage is above or below the level of the positive voltage.
This functionality is required in portable systems where all of the sub-systems operate off of the battery pack, which has a positive voltage. There is no negative voltage source in the system, so it must be created from the batteries.
Figure 1a shows the basic schematic for the inverting buck-boost converter. The battery supplies the input voltage at VIN and there are two MOSFETs, followed by an inductor, output capacitor, and the load.
This arrangement is nearly identical to the buck converter except that the output voltage node is grounded and the buck converter’s ground node is the negative output voltage. This symmetry of configuration allows most buck converters to be used in the inverting buck-boost topology, provided that the input voltage and output current ratings of the buck converter are sufficient for the particular inverting buck-boost application.
Figure 1. Inverting buck-boost converter operating states.
Figures 1b and 1c show the inverting buck-boost converter in its two states. In Figure 1b, the control MOSEFT, Q1, is on (closed) and the sync MOSFET, Q2, is off (open). During this state, the inductor charges with current while the output voltage and current is supplied by the output capacitor only. In Figure 1c, the control MOSFET opens and the sync MOSFET closes, allowing the inductor current to be delivered to the load and recharge the output capacitor.
Since the inductor is only connected to the load during the second state (Figure 1c), the inductor must be charged with a higher current in the first state (Figure 1b). This provides the load current and recharges the output capacitor during the time when the sync MOSFET is closed. Thus, the inductor current is greater than the load current.
Also note that in Figure 1a, the voltage across the MOSFETs is VIN to the negative output voltage, –VOUT. Since the MOSFETs typically are integrated inside the power converter to keep the solution size small, the power converter device sees a voltage greater than VIN across its terminals. It sees VIN to –VOUT.
The increased VIN rating required and higher inductor current are two important effects to be considered when using a buck converter in the inverting buck-boost topology.
Three inverting buck-boost solutions
The input source for the typical portable system is a one- or two-cell lithium battery, which have nominal voltages of 3.6V or 7.2V, respectively. This battery pack voltage varies from around 3V to 4.2V for a single-cell pack, and from 6V to 8.4V for a two-cell pack.
Typically, the LCD panel needs a negative voltage around –5V. So the power converter supplying this –5V rail needs an input voltage rating of at least 13.4V (8.4V – (–5V)). Three 17 V-rated power converters are compared. Each emphasizes one or more aspects of a good design: small size, low component count, efficiency, and design simplicity.