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LTC3409A Datasheet(PDF) 11 Page - Linear Technology |
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LTC3409A Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC3409A 11 3409af APPLICATIONS INFORMATION The output ripple ΔVOUT is determined by: ΔVOUT =ΔIL ESR+ 1 8• f • COUT ⎛ ⎝⎜ ⎞ ⎠⎟ where f = operating frequency, COUT = output capacitance and ΔIL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since ΔIL increases with input voltage. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. In the case of tantalum, 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 value, lower cost ceramic capacitors are now avail- able in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the LTC3409A’s control loop does not depend on the output capacitor’s ESR for stable operation, ceramic capacitors can be used to achieve very low output ripple and small circuit size. However, care must be taken when these 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 Equation 2: VOUT = 0.612V 1+ R1 R2 ⎛ ⎝⎜ ⎞ ⎠⎟ (2) The external resistive divider is connected to the output, allowing remote voltage sensing as shown in Figure 1. VFB VOUT R1 R2 3409A F01 GND LTC3409A Figure 1 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 + ...) where L1, L2, etc. are the individual losses as a percent- age of input power. Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses in LTC3409A circuits: VIN quiescent current and I2R losses. The VIN quiescent current loss dominates the efficiency loss at very low load currents whereas the I2R loss dominates the efficiency loss at medium to high load currents. In a typical efficiency plot, the efficiency curve at very low load currents can be misleading since the actual power lost is of no consequence as illustrated in Figure 2. |
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