Use DS3882 CCFL controller to drive the common circuit of two lights

Introduction

The DS3882 is a two-channel controller for cold-cathode fluorescent lamps (CCFL) that backlight liquid crystal displays (LCD) in automotive, industrial, and avionic applications. In some of these applications, the LCD panels contain two adjacent CCFL lamps that are mounted on one side of the panel. These arrangements typically use a common low-voltage, side lamp connection. This configuration presents a system challenge because the normal scheme of sensing lamp current on the low-voltage side of the lamp cannot be implemented when both lamps share a common return.
This application note describes how to use a CCFL controller to drive two CCFL lamps that share a common return. The article guides the practical implementation for this type of drive arrangement. The DS3882 CCFL controller will be used as the controller in the example.

Two-Lamp, Common-Return-Drive Scheme

The drive scheme shown in the DS3882's data sheet applies when access to the low-voltage side of each lamp is available. Driving two CCFL lamps that share a common low-voltage return, however, requires a different configuration. Figure 1 shows how the CCFL lamps are arranged in an LCD panel. This alternate drive scheme uses the DS3882 to provide full protection for both lamps and adds only a few extra passive components. The unique design elements of this example configuration will be discussed in detail.