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ISL97644IRZ-TK Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL97644IRZ-TK Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 15 page 11 FN9228.1 December 14, 2007 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 used to match the internal slope compensation. The inductor must be able to handle the following average and peak current: Some inductors are recommended in Table 3. Rectifier Diode A high-speed diode is necessary due to the high switching frequency. Schottky diodes are recommended because of their fast recovery time and low forward voltage. The reverse voltage rating of this diode should be higher than the maximum output voltage. The rectifier diode must meet the output current and peak inductor current requirements. The following table is some recommendations for boost converter diode. Output Capacitor The output capacitor supplies the load directly and reduces the ripple voltage at the output. Output ripple voltage consists of two components: the voltage drop due to the inductor ripple current flowing through the ESR of output capacitor, and the charging and discharging of the output capacitor. For low ESR ceramic capacitors, the output ripple is dominated by the charging and discharging of the output capacitor. The voltage rating of the output capacitor should be greater than the maximum output voltage. Note: Capacitors have a voltage coefficient that makes their effective capacitance drop as the voltage across then increases. COUT in the equation above assumes the effective value of the capacitor at a particular voltage and not the manufacturer’s stated value, measured at zero volts. The following table shows some selections of output capacitors. Compensation The boost converter of ISL97644 can be compensated by a RC network connected from CM1 pin to ground. 4.7nF and 10k RC network is used in the demo board. The larger value resistor and lower value capacitor can lower the transient overshoot, however, at the expense of stability of the loop. 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. Linear-Regulator (LDO) The ISL97644 includes a LDO with adjustable output, and it can supply current up to 350mA. The output voltage is adjusted by connection of ADJ pin. When ADJ pin is connected to ground, the output voltage is set to 3.3V; when ADJ pin is floating, the output voltage is set to 2.85V, and when ADJ pin is connected to LDO_OUT pin, the output voltage is set to 2.5V. TABLE 3. BOOST INDUCTOR RECOMMENDATION INDUCTOR DIMENSIONS (mm) MFG PART NUMBER 6.8µH/3APEAK 7.3x6.8x3.2 TDK RLF7030T-6R8N3R0 10µH/4APEAK 8.3X8.3X4.5 Sumida CDR8D43-100NC 5.2µH/4.55APEAK 10x10.1x3.8 Cooper Bussmann CD1-5R2 TABLE 4. BOOST CONVERTER RECTIFIER DIODE RECOMMENDATION DIODE VR/IAVG RATING PACKAGE MFG SS23 30V/2A SMB Fairchild Semiconductor MBRS340 40V/3A SMC International Rectifier SL23 30V/2A SMB Vishay Semiconductor I LAVG I O 1D – ------------- = I LPK I LAVG ΔI L 2 -------- + = (EQ. 5) V RIPPLE I LPK ESR V O V IN – V O ------------------------ I O C OUT ---------------- 1 f s ---- × × + × = (EQ. 6) TABLE 5. BOOST OUTPUT CAPACITOR RECOMMENDATION CAPACITOR SIZE MFG PART NUMBER 10µF/25V 1210 TDK C3225X7R1E106M 10µF/25V 1210 Murata GRM32DR61E106K FIGURE 15. CASCADED MOSFET TOPOLOGY FOR HIGH OUTPUT VOLTAGE APPLICATIONS Intersil ISL97644 LX FB AVDD VIN ISL97644 |
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