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EL7585ILZ Datasheet(PDF) 9 Page - Intersil Corporation |
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EL7585ILZ Datasheet(HTML) 9 Page - Intersil Corporation |
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9 / 19 page  9 FN7345.2 March 9, 2006 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 results in a compact, low cost power system for LCD panel design. The EL7585 is 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 the switching MOSFET. Figure 19 shows the block diagram of the boost regulator. 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: The current through the MOSFET is limited to 3.5A peak. This restricts the maximum output current based on the following equation: Where ∆IL is peak to peak inductor ripple current, and is set by: where fS is the switching frequency. AVDD VIN ---------------- 1 1D – ------------- = AVDD R1 R2 + R1 --------------------- VREF × = IOMAX ILMT ∆I L 2 -------- – VIN VO --------- × = ∆I L VIN L --------- D fS ----- × = Iref Iref FBB Ifb Ifb CINT VOLTAGE AMPLIFIER LX PGND SHUTDOWN & START-UP CONTROL GM AMPLIFIER SLOPE COMPENSATION BUFFER PWM LOGIC CURRENT AMPLIFIER CLOCK REFERENCE GENERATOR FIGURE 19. BLOCK DIAGRAM OF THE BOOST REGULATOR EL7585 |
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