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EL7520ILZ-T13 Datasheet(PDF) 10 Page - Intersil Corporation |
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EL7520ILZ-T13 Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 18 page 10 FN7318.0 July 12, 2005 EL7520, EL7520A Boost Converter The main boost converter is a current mode PWM controller operating at a fixed frequency. The 1MHz switching frequency enables the use of low profile inductor and multilayer ceramic capacitors, which results in a compact, low-cost power system for LCD panel design. The boost converter can operate in continuous or discontinuous inductor current mode. The EL7520 and EL7520A are designed for continuous current mode, but they can also operate in discontinuous current mode at light load. In continuous current mode, current flows continuously in the inductor during the entire switching cycle in steady state operation. The voltage conversion ratio in continuous current mode is given by: Where D is the duty cycle of switching MOSFET. Figure 25 shows the function diagram of the boost controller. It uses a summing amplifier architecture consisting of GM stages for voltage feedback, current feedback and slope compensation. A comparator looks at the peak inductor current cycle by cycle and terminates the PWM cycle if the current limit is reached. An external resistor divider is required to divide the output voltage down to the nominal reference voltage. Current drawn by the resistor network should be limited to maintain the overall converter efficiency. The maximum value of the resistor network is limited by the feedback input bias current and the potential for noise being coupled into the feedback pin. A resistor network in the order of 60k Ω is recommended. The boost converter output voltage is determined by the following equation: Σ PWM LOGIC CONTROLLER BUFFER CURRENT LIMIT REF GENERATOR OSCILLATOR SLOPE COMPENSATION OSC REFERENCE GENERATOR VREF LX DRVB ISIN ISAD GM AMPLIFIER UVLO COMPARATOR VOLTAGE AMPLIFIER CURRENT AMPLIFIER CURRENT LIMIT COMPARATOR THERMAL SHUTDOWN SS + - UVLO COMPARATOR BUFFER UVLO COMPARATOR UVLO COMPARATOR SS + - SS + - BUFFER SHUTDOWN & STARTUP CONTROL BUFFER FBP DRVL FBL DRVP FBB CINT EN DRVN FBN 0.4V 0.2V VREF VREF FIGURE 24. BLOCK DIAGRAM VBOOST VIN ------------------------ 1 1D – ------------- = VBOOST R1 R2 + R1 --------------------- VREF × = |
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