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NR30102R2M Datasheet(PDF) 9 Page - Linear Technology |
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NR30102R2M Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page 9 LTC3410 3410fb VFB GND LTC3410 0.8V ≤ VOUT ≤ 5.5V R2 R1 3410 F02 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. The recommended capacitance value to use is 4.7 µF for both input and output capacitor. For applications with VOUT greater than 2.5V, the recommended value for output capacitance should be increased. See Table 2. Table 2. Capacitance Selection OUTPUT OUTPUT INPUT VOLTAGE RANGE CAPACITANCE CAPACITANCE 0.8V ≤ VOUT ≤ 2.5V 4.7 µF4.7µF VOUT > 2.5V 10 µH or 2x 4.7µF4.7µF Output Voltage Programming (LTC3410 Only) The output voltage is set by a resistive divider according to the following formula: VV R R OUT =+ ⎛ ⎝⎜ ⎞ ⎠⎟ 08 1 2 1 2 .( ) The external resistive divider is connected to the output, allowing remote voltage sensing as shown in Figure 2. Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% – (L1 + L2 + L3 + ...) 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 LTC3410’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 Figure 2. Setting the LTC3410 Output Voltage APPLICATIO S I FOR ATIO |
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