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ISL97650 Datasheet(PDF) 13 Page - Renesas Technology Corp |
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ISL97650 Datasheet(HTML) 13 Page - Renesas Technology Corp |
13 / 20 page ISL97650 FN9198 Rev.4.00 Page 13 of 20 April 17, 2009 Cascaded MOSFET Application An 20V N-channel MOSFET is integrated in the boost regulator. For the applications where the output voltage is greater than 20V, an external cascaded MOSFET is needed as shown in Figure 15. The voltage rating of the external MOSFET should be greater than AVDD. Buck Converter The buck converter is the step down converter, which supplies the current to the logic circuit of the LCD system. The ISL97650 integrates an 20V N-channel MOSFET to save cost and reduce external component count. In the continuous current mode, the relationship between input voltage and output voltage is as shown in Equation 10: Where D is the duty cycle of the switching MOSFET. Because D is always less than 1, the output voltage of buck converter is lower than input voltage. The peak current limit of buck converter is set to 2A, which restricts the maximum output current (average) based on Equation 11: Where Ipp is the ripple current in the buck inductor as Equation 12, Where L is the buck inductor, fs is the switching frequency (1.2MHz). Feedback Resistors The buck converter output voltage is determined by the Equation 13: Where R14 and R15 are the feedback resistors of buck converter to set the output 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 1k is recommended. Buck Converter Input Capacitor The capacitor should support the maximum AC RMS current which happens when D = 0.5 and maximum output current. Where Io is the output current of the buck converter. Table 6 shows some recommendations for input capacitors. Buck Inductor An 3.3µH to 10µH inductor is the good choice for the buck converter. Besides the inductance, the DC resistance and the saturation current are also the factor needed to be considered when choosing buck inductor. Low DC resistance can help maintain high efficiency, and the saturation current rating should be 2A. Here are some recommendations for buck inductor. FIGURE 15. CASCADED MOSFET TOPOLOGY FOR HIGH OUTPUT VOLTAGE APPLICATIONS INTERSIL ISL97650 LX1, LX2 FBB AVDD VIN VLOGIC VIN ---------------------- D = (EQ. 10) IOMAX 2A I pp – = (EQ. 11) I pp VLOGIC Lfs ---------------------- 1D – = (EQ. 12) TABLE 6. INPUT CAPACITOR (BUCK) RECOMMENDATION CAPACITOR SIZE VENDOR PART NUMBER 10µF/16V 1206 TDK C3216X7R1C106M 10µF/10V 0805 Murata GRM21BR61A106K 22µF/16V 1210 Murata C3225X7R1C226M TABLE 7. BUCK INDUCTOR RECOMMENDATION INDUCTOR DIMENSIONS (mm) VENDOR PART NUMBER 4.7µH/ 2.7APEAK 5.7x5.0x4.7 Murata LQH55DN4R7M01K 6.8µH/ 3APEAK 7.3x6.8x3.2 TDK RLF7030T-6R8M2R8 10µH/ 2.4APEAK 12.95x9.4x3.0 Coilcraft DO3308P-103 VLOGIC R14 R15 + R15 --------------------------- VFBL = (EQ. 13) Iacrms CIN D1 D – IO = (EQ. 14) |
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