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LTC3560ES6LTCFY Datasheet(PDF) 9 Page - Linear Technology |
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LTC3560ES6LTCFY Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LTC3560 3560f APPLICATIO S I FOR ATIO more than powdered iron core inductors with similar electrical characteristics. The choice of which style induc- tor to use often depends more on the price vs size require- ments and any radiated field/EMI requirements than on what the LTC3560 requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3560 applications. CIN and COUT Selection In continuous mode, the source current of the top MOS- FET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maxi- mum RMS capacitor current is given by: CI VV V V IN OMAX OUT IN OUT IN required IRMS ≅ − () []12/ This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is com- monly used for design because even significant deviations do not offer much relief. Note that the capacitor manufacturer’s ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Always consult the manufac- turer if there is any question. The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR require- ment for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. The output ripple ∆VOUT is determined by: ∆≅ ∆ + ⎛ ⎝⎜ ⎞ ⎠⎟ V I ESR fC OUT L OUT 1 8 where f = operating frequency, COUT = output capacitance and ∆IL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since ∆IL increases with input voltage. If tantalum capacitors are used, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalum. These are specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP, Kemet T510 and T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations. Using Ceramic Input and Output Capacitors Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the LTC3560’s control loop does not depend on the output capacitor’s ESR for stable operation, ceramic capacitors can be used freely to achieve very low output ripple and small circuit size. However, care must be taken when ceramic capacitors are used at the input and the output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN, large enough to damage the part. When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best temperature and voltage charac- teristics of all the ceramics for a given value and size. Output Voltage Programming The output voltage is set by a resistive divider according to the following formula: VV R R OUT =+ ⎛ ⎝⎜ ⎞ ⎠⎟ 06 1 2 1 . (2) The external resistive divider is connected to the output, allowing remote voltage sensing as shown in Figure 2. |
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