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LTC3403 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3403 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page 11 LTC3403 3403f 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 LTC3403’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. Ceramic capacitors of Y5V material are not recommended because normal operating voltages cause their bulk ca- pacitance to become much less than the nominal value. Programming the Output Voltage With a DAC The output voltage can be dynamically programmed to any voltage from 0.3V to 3.5V with an external DAC driving the REF pin. When the output is commanded low, the output voltage descends quickly in forced continuous mode pulling current from the output and transferring it to the input. If the input is not connected to a low impedance source capable of absorbing the energy, the input voltage could rise above the absolute maximum voltage of the part and get damaged. The faster VOUT is commanded low, the higher is the voltage spike at the input. For best results, ramp the REF pin from high to low as slow as the application will allow. Avoid abrupt changes in voltage of >0.2V/ µs. If ramp control is unavailable, an RC filter with a time constant of 10 µs can be inserted between the REF pin and the DAC as shown in Figure 3. 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 percentage of input power. Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses in LTC3403 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 little consequence as illustrated in Figure 4. APPLICATIO S I FOR ATIO LTC3403 REF GND DAC 10k 1000pF Figure 3. Filtering the REF Pin Figure 4. Power Lost vs Load Current LOAD CURRENT (mA) 0.1 1 0.00001 0.001 1 10 100 1000 3406 F04 0.0001 0.01 0.1 VOUT = 1.2V VOUT = 1.5V VOUT = 1.8V VOUT = 2.5V |
Similar Part No. - LTC3403_15 |
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Similar Description - LTC3403_15 |
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