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ISL97650 Datasheet(PDF) 11 Page - Renesas Technology Corp |
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ISL97650 Datasheet(HTML) 11 Page - Renesas Technology Corp |
11 / 20 page ISL97650 FN9198 Rev.4.00 Page 11 of 20 April 17, 2009 Applications Information The ISL97650 provides a complete power solution for TFT LCD applications. The system consists of one boost converter to generate AVDD voltage for column drivers, one buck converter to provide voltage to logic circuit in the LCD panel, one integrated VON charge pump and one VOFF linear- regulator controller to provide the voltage to row drivers. This part also integrates VON-slice circuit which can help to optimize the picture quality. With the high output current capability, this part is ideal for big screen LCD TV and monitor panel application. The integrated boost converter and buck converter operate at 1.2MHz which can allow to use multilayer ceramic capacitors and low profile inductor which result in low cost, compact and reliable system. The logic output voltage is independently enabled to give flexibility to the system designers. Boost Converter The boost converter is a current mode PWM converter operating at a fixed frequency of 1.2MHz. It can operate in both discontinuous conduction mode (DCM) at light load and continuous mode (CCM). 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 Equation 1: Where D is the duty cycle of the switching MOSFET. The boost converter 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 Equation 2: The current through the MOSFET is limited to 2.6Apeak. This restricts the maximum output current (average) based on Equation 3: Where IL is peak to peak inductor ripple current, and is set by Equation 4: where fs is the switching frequency(1.2MHz). Table 1 gives typical values (margins are considered 10%, 3%, 20%, 10% and 15% on VIN, VO, L, fs and IOMAX): The minimum duty cycle of the ISL97650 is 25%. When the operating duty cycle is lower than the minimum duty cycle, the part will not switch in some cycles randomly, which will cause some LX pulses to be skipped. In this case, LX pulses are not consistent any more, but the output voltage (AVDD) is still regulated by the ratio of R3 and R5. This relationship is given by Equation 2. Because some LX pulses are skipped, the ripple current in the inductor will become bigger. Under the worst case, the ripple current will be from 0 to the threshold of the current limit. In turn, the bigger ripple current will increase the output voltage ripple. Hence, it will need more output capacitors to keep the output ripple at the same level. When the input voltage equals, or is larger than, the output voltage, the boost converter will stop switching. The boost converter is not regulated any more, but the part will still be on and other channels are still regulated. The typical waveforms of pulse- skipping mode are shown in the section “Typical Performance Curves” on page 5. Boost Converter Input Capacitor An input capacitor is used to suppress the voltage ripple injected into the boost converter. The ceramic capacitor with capacitance larger than 10µF is recommended. The voltage rating of input capacitor should be larger than the maximum input voltage. Some capacitors are recommended in Table 2 for input capacitor. Boost Inductor The boost inductor is a critical part which influences the output voltage ripple, transient response, and efficiency. Values of 3.3µH to 10µH are to match the internal slope compensation. Vboost VIN ------------------ 1 1D – ------------- = (EQ. 1) AVDD R3 R5 + R5 --------------------- VFBB = (EQ. 2) IOMAX ILMT I L 2 -------- – VIN VO --------- = (EQ. 3) TABLE 1. MAXIMUM OUTPUT CURRENT CALCULATION VIN (V) VO (V) L (µH) fs (MHz) IOMAX (mA) 5 9 6.8 1.2 1138 5 12 6.8 1.2 777 4 15 6.8 1.2 560 12 15 6.8 1.2 1345 12 18 6.8 1.2 998 TABLE 2. BOOST CONVERTER INPUT CAPACITOR RECOMMENDATION CAPACITOR SIZE VENDOR PART NUMBER 10µF/25V 1210 TDK C3225X7R1E106M 10µF/25V 1210 Murata GRM32DR61E106K I L VIN L --------- D fS ----- = (EQ. 4) |
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