Cold Cathode Fluorescent Lamps (CCFLs) offer the highest available efficiency for backlighting an LCD display, making them a popular choice for handheld devices. The CCFL controller must also be efficient to conserve battery power, and small, to fit in the tight spaces required by the latest handheld electronic devices. Figure 1 shows a compact and efficient 1W CCFL controller using the LTC1697 synchronous current mode controller.
To control the CCFL brightness, the LTC1697 implements an internal PWM dimming scheme on the CCFL current—a method that is both efficient and offers the widest dimming range. A single capacitor, C5, determines the PWM frequency and the LTC1697 controller can implement a smooth transition from zero to full brightness without any hysteresis or “pop-on.” The dimming input is simply a 1V to 2V signal at VDIM, corresponding to 0% to 100% CCFL brightness, where the maximum CCFL current (100% brightness) is set by R3. The dimming control input can also be a digital PWM signal.
The circuit runs off a single Li-Ion battery (2.7V– 5.5V) and the LTC1697 consumes less than 1µA when in Shutdown mode. The LTC1697 also incorporates Open Lamp detection to protect the CCFL transformer, T1, from excessively high voltages.