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EL7641ILTZ Datasheet(PDF) 10 Page - Renesas Technology Corp |
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EL7641ILTZ Datasheet(HTML) 10 Page - Renesas Technology Corp |
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10 / 19 page  EL7640, EL7641, EL7642 FN7415 Rev 2.00 Page 10 of 19 February 22, 2006 FIGURE 17. BLOCK DIAGRAM Boost Converter The main boost converter is a current mode PWM converter operating at a fixed frequency. The 1.2MHz 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 EL7640, EL7641, EL7642 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 18 shows the block 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 is recommended. The boost converter output voltage is determined by the following equation: PWM LOGIC CONTROLLER BUFFER OSCILLATOR SLOPE COMPENSATION OSC REFERENCE GENERATOR VREF GM AMPLIFIER UVLO COMPARATOR VOLTAGE AMPLIFIER CURRENT AMPLIFIER THERMAL SHUTDOWN SS + - UVLO COMPARATOR BUFFER UVLO COMPARATOR SS + - SHUTDOWN & START-UP CONTROL BUFFER FBP DRVP FBB CINT DRVN FBN 0.4V 0.2V VREF COMP CURRENT LIMIT COMPARATOR CURRENT REF PGND LX VBOOST VIN ------------------------ 1 1D – ------------- = VBOOST R1 R2 + R1 --------------------- VREF = |
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