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ISL97651 Datasheet(PDF) 12 Page - Renesas Technology Corp |
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ISL97651 Datasheet(HTML) 12 Page - Renesas Technology Corp |
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12 / 20 page  ISL97651 FN7493 Rev 3.00 Page 12 of 20 April 24, 2009 Some inductors are recommended in Table 3. Rectifier Diode (Boost Converter) 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. Table 4 shows 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 Equation 6 assumes the effective value of the capacitor at a particular voltage and not the manufacturer’s stated value, measured at 0V. Table 5 shows some selections of output capacitors. PI Loop Compensation (Boost Converter) The boost converter of ISL97651 can be compensated by a RC network connected from CM1 pin to ground. C3 = 4.7nF and R1 = 10k RC network is used in the demo board. A higher resistor value can be used to lower the transient overshoot - however, this may be at the expense of stability to the loop. The stability can be examined by repeatedly changing the load between 100mA and a max level that is likely to be used in the system being used. The AVDD voltage should be examined with an oscilloscope set to AC 100mV/div and the amount of ringing observed when the load current changes. Reduce excessive ringing by reducing the value of the resistor in series with the CM1 pin capacitor. Boost Converter Feedback Resistors and Capacitor An RC network across feedback resistor R5 may be required to optimize boost stability when AVDD voltage is set to less than 12V. This network reduces the internal voltage feedback used by the IC. This RC network sets a pole in the control loop. This pole is set to approximately fp = 10kHz for COUT = 10µF and fp = 4kHz for COUT = 30µF. Alternatively, adding a small capacitor (20pF to 100pF) in parallel with R5 (i.e. R17 = short) may help to reduce AVDD noise and improve regulation, particularly if high value feedback resistors are used. 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 12. The voltage rating of the external MOSFET should be greater than AVDD. TABLE 3. BOOST INDUCTOR RECOMMENDATION INDUCTOR DIMENSIONS (mm) VENDOR PART NUMBER 6.8µH/ 4.6APEAK 12.95x9.4x5.21 Coilcraft DO3316P-682ML 10µH/ 5.5APEAK 10x10x5 Sumida CDR10D48MNNP-100NC 5.2µH/ 4.55APEAK 10x10.1x3.8 Cooper Bussmann CD1-5R2 TABLE 4. BOOST CONVERTER RECTIFIER DIODE RECOMMENDATION DIODE VR/IAVG RATING PACKAGE VENDOR SS23 30V/2A SMB Fairchild Semiconductor MBRS340 40V/3A SMC International Rectifier SL23 30V/2A SMB Vishay Semiconductor VRIPPLE ILPK ESR VO VIN – VO ------------------------ IO COUT ---------------- 1 fs ---- + = (EQ. 6) TABLE 5. BOOST OUTPUT CAPACITOR RECOMMENDATION CAPACITOR SIZE VENDOR PART NUMBER 10µF/25V 1210 TDK C3225X7R1E106M 10µF/25V 1210 Murata GRM32DR61E106K R17 1 0.1 R5 ------------------------- 1 R3 ---------- – 1 – = (EQ. 7) C18 1 2 3.142 fp R5 ------------------------------------------------------- = (EQ. 8) |
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