EL7585A stands for multiple output regulator used in all large panel, TFT-LCD applications. It features a single boost converter integrating a 3.5A FET, two positive LDOs for the VON and V logic generation, and a single negative LDO for the V OFF generation. The boost converter can be programmed to operate in either P-mode or PI-mode to improve load regulation. The EL7585A also integrates all four fault protection channels. Once a fault is detected, the device is locked off until input power or EN is cycled. The device also features an integrated boot sequence for V boost, V off, then VON or V off, VBOOST and VON sequencing. The latter requires an external transistor. The timing of the startup sequence is set using an external capacitor. The V logic output is constantly enabled, but is turned off when a fault condition is detected. The EL7585A is specified over the -40°C to +85°C temperature range.

Application Information

The EL7585A is a highly integrated multi-output power supply solution for TFT-LCD applications. The system consists of a high-efficiency boost converter and three linear-regulator controllers (VON, Vshutdown and Vlogic) with multiple protection functions. The block diagram is shown in Figure 1. The EL7585A integrates an N-channel MOSFET boost converter to minimize external component count and cost. The AVDD, VON, Vshutdown and V logic output voltages are individually set by external resistors. VON, V off voltage requires an external charge pump which is regulated after using the integrated LDO controller.

figure 1

boost converter

The main boost converter is a current-mode PWM converter at a fixed frequency of 1MHz, which enables the use of low-profile inductors and multilayer ceramic capacitors. This has resulted in a compact, low-cost power system for LCD panel designs. The EL7585A are designed for continuous current mode, but they can also operate in discontinuous current mode, under lamp duty. In continuous current mode, current flows continuously, and the inductor operates in a stable state throughout the switching cycle. In continuous voltage conversion ratio current mode is given by:

where D is the duty cycle of the switching MOSFET. Figure 2 shows the block diagram of the boost regulator. It uses a summing amplifier architecture, compensated by gram voltage feedback, current feedback and slope stages. The comparator looks at the peak inductor current loop by loop and terminates the PWM cycle if the current limit is reached.

figure 2

An external resistor divider is required to divide the output voltage down to the nominal reference voltage. The current draw by the resistor network should be limited to maintaining the efficiency of the entire converter. The maximum value of the resistor network is limited by the feedback input bias current and the potential for noise is coupled to the feedback pin. A resistor network in the order of 60KΩ is recommended. The output voltage of the boost converter is determined by the following equation:

Current is limited to 3.5A peak through the MOSFET. This limits the following equation based on the maximum output current: