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MAX5027 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX5027 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 13 page 500kHz, 36V Output, SOT23, PWM Step-Up DC-DC Converters 10 ______________________________________________________________________________________ For a design in which VIN = 5V, VOUT = 30V, IOUT = 4mA, η = 0.5, and TS = 2µs, LUPPER = 87µH and LLOWER = 12µH. For a worst-case scenario in which VIN = 4.75V, VOUT = 29V, IOUT = 4.4mA, η = 0.5, and TS = 1.25µs, LUPPER = 46µH and LLOWER = 9µH. The choice of 47µH as the recommended inductance value is reasonable given the worst-case scenario above. In general, the higher the inductance, the lower the switching noise. Load regulation is also better with higher inductance. Diode Selection The MAX5025–MAX5028’s high switching frequency demands a high-speed rectifier. Schottky diodes are recommended for most applications because of their fast recovery time and low forward-voltage drop. Ensure that the diode’s peak current rating is greater than or equal to the peak inductor current. Also, the diode reverse breakdown voltage must be greater than VOUT. Table 2 lists diode vendors. Capacitor Selection Output Filter Capacitor The output filter capacitor should be 1µF or greater. To achieve low output ripple, a capacitor with low ESR, low ESL, and high capacitance value should be selected. For very low output ripple applications, the output of the boost converter can be followed by an RC filter to fur- ther reduce the ripple. Figure 3 shows a 100 Ω, 1µF fil- ter used to reduce the switching output ripple to 1mVp-p. X7R ceramic capacitors are better for this boost appli- cation because of their low ESR and tighter tolerance over temperature than the Y5V ceramic capacitors. Table 3 below lists manufacturers of recommended capacitors. Input Capacitor Bypass VCC with a 4.7µF ceramic capacitor as close to the IC as is practical. Applications Information Layout Considerations The MAX5025–MAX5028 switch at high speed, man- dating careful attention to layout for optimum perfor- mance. Protect sensitive analog grounds by using a star ground configuration. Minimize ground noise by connecting GND, PGND, the input bypass-capacitor ground lead, and the output-filter ground lead to a sin- gle point (star ground configuration). Also, minimize Table 2. Schottky Diode Vendors VENDOR PHONE FAX PART NUMBERS Comchip 510-657-8671 510-657-8921 CDBS1045 Panasonic 408-942-2912 408-946-9063 MA2Z785 ST-Microelectronics 602-485-6100 602-486-6102 TMMBAT48 Vishay-Telefunken 402-563-6866 402-563-6296 BAS382 Zetex 631-360-2222 631-360-8222 ZHCS500 Table 3. Capacitor Table COMPANY PHONE FAX PART NUMBERS GRM42-2X7R105K050AD (1µF capacitor) Murata 814-237-1431 814-238-0490 GRM32-1210R71C475R (4.7µF capacitor) UMK325BJ105KH (1µF capacitor) Taiyo Yuden 408-573-4150 408-573-4159 EMK316BJ475ML (4.7µF capacitor) C3225X7R1H155K (1.5µF capacitor) TDK 847-803-6100 847-803-6296 C3225X7R1H105K (1µF capacitor) |
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