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LT3463A Datasheet(PDF) 6 Page - Linear Technology |
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LT3463A Datasheet(HTML) 6 Page - Linear Technology |
6 / 8 page 6 LT3463/LT3463A 3463f inrush current include a larger more abrupt voltage step at VIN, a larger output capacitor tied to the outputs, and an inductor with a low saturation current. While the internal diode is designed to handle such events, the inrush current should not be allowed to exceed 1 amp. For circuits that use output capacitor values within the recommended range and have input voltages of less than 5V, inrush current remains low, posing no hazard to the device. In cases where there are large steps at VIN and/or a large capacitor is used at the outputs, inrush current should be measured to ensure safe operation. Setting the Output Voltages The output voltages are programmed using two feedback resistors. As shown in Figure 1, resistors R1 and R2 program the positive output voltage (for Switcher 1), and resistors R3 and R4 program the negative output voltage (for Switcher 2) according to the following formulas: VV R R VV R R OUT OUT 1 2 125 1 2 1 125 4 3 =+ = . –. R1 and R3 are typically 1% resistors with values in the range of 50k to 250k. Board Layout Considerations As with all switching regulators, careful attention must be paid to the PCB board layout and component placement. To maximize efficiency, switch rise and fall times are made as short as possible. To prevent electromagnetic interfer- ence (EMI) problems, proper layout of the high frequency switching path is essential. The voltage signal of the SW pin has sharp rising and falling edges. Minimize the length and area of all traces connected to the SW pin and always use a ground plane under the switching regulator to minimize interplane coupling. In addition, the ground connection for the feedback resistor R1 should be tied directly to the GND pin and not shared with any other component, ensuring a clean, noise-free connection. APPLICATIO S I FOR ATIO output voltages from a high input voltage source will often exceed the 50V maximum switch rating. For instance, a 12V to – 40V converter using the inverting topology would generate 52V on the SW pin, exceeding its maximum rating. For this application, an inverting charge pump is the best topology. The formula below calculates the approximate inductor value to be used for an inverting charge pump regulator using the LT3463. As for the boost inductor selection, a larger or smaller value can be used. For designs with varying VIN such as battery powered applications, use the minimum VIN value in the equation below. L VV V I t OUT IN MIN D LIM OFF = −+ () Capacitor Selection The small size and low ESR of ceramic capacitors makes them ideal for LT3463 applications. Use only X5R and X7R types because they retain their capacitance over wider voltage and temperature ranges than other ceramic types. A 1 µF input capacitor and a 0.22µF or 0.47µF output capacitor are sufficient for most applications. Table 2 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers for more detailed information on their entire selection of ceramic capacitors. For appli- cations needing very low output voltage ripple, larger output capacitor values can be used. Table 2. Recommended Ceramic Capacitor Manufacturers MANUFACTURER PHONE URL AVX 843-448-9411 www.avxcorp.com Kemet 408-986-0424 www.kemet.com Murata 814-237-1431 www.murata.com Taiyo Yuden 408-573-4150 www.t-yuden.com Inrush Current When VIN is increased from ground to operating voltage while the output capacitor is discharged, an inrush current will flow through the inductor and integrated Schottky diode into the output capacitor. Conditions that increase |
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