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ISL97651ARTZ-T Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL97651ARTZ-T Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 19 page  11 FN7493.2 March 15, 2007 Applications Information The ISL97651 provides a complete power solution for TFT LCD applications. The system consists of one boost converter to generate the 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 the use of 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 Figure 11 shows the functional 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 Equation 2: The current through the MOSFET is limited to a minimum of 4.4APEAK (maximum values can be up to 6.3APEAK. This restricts the maximum output current (average) based on Equation 3: Where ΔIL 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: Boost Converter Input Capacitor An input capacitor is used to suppress the voltage ripple injected into the boost converter. A ceramic capacitor with capacitance larger than 10µF is recommended. The voltage rating of input capacitor should be larger than the maximum input voltage. Examples of recommended capacitors are given in Table 2 below. Boost Inductor The boost inductor is a critical component which influences the output voltage ripple, transient response, and efficiency. Values of 3.3µH to 10µH are to match the internal slope compensation. The inductor must be able to handle without saturating the following average and peak current: V BOOST V IN ------------------------ 1 1D – ------------- = (EQ. 1) V BOOST R 3 R 5 + () R 5 -------------------------- V REF × = (EQ. 2) TABLE 1. MAXIMUM OUTPUT CURRENT CALCULATION VIN (V) VO (V) L (µH) FS (MHz) IOMAX (mA) 4 9 6.8 1.2 1661 4 12 6.8 1.2 1173 4 15 6.8 1.2 879 5 9 6.8 1.2 2077 5 12 6.8 1.2 1466 5 15 6.8 1.2 1099 TABLE 2. BOOST CONVERTER INPUT CAPACITOR RECOMMENDATION CAPACITOR SIZE VENDOR PART NUMBER 10µF/16V 1206 TDK C3216X7R1C106M 10µF/10V 0805 Murata GRM21BR61A106K 22µF/10V 1210 Murata GRB32ER61A226K I OMAX I LMT ΔI L 2 -------- – ⎝⎠ ⎛⎞ VIN V O --------- × = (EQ. 3) ΔI L V IN L --------- D f S ----- × = (EQ. 4) I LAVG I O 1D – ------------- = I LPK I LAVG ΔI L 2 -------- + = (EQ. 5) ISL97651 |
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