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LTC3416 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3416 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LTC3416 9 3416f APPLICATIO S I FOR ATIO prevent large voltage transients from occurring, a low ESR input capacitor sized for the maximum RMS current should be used. The maximum RMS current is given by: II V V V V RMS OUT MAX OUT IN IN OUT = () –1 This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher tempera- ture than required. Several capacitors may also be paral- leled to meet size or height requirements in the design. For low input voltage applications, sufficient bulk input ca- pacitance is needed to minimize transient effects during output load changes. The selection of COUT is determined by the effective series resistance (ESR) that is required to minimize voltage ripple and load step transients as well as the amount of bulk capacitance that is necessary to ensure that the control loop is stable. Loop stability can be checked by viewing the load transient response as described in a later section. The output ripple, ∆VOUT, is determined by: ∆≤ ∆ + V I ESR fC OUT L OUT 1 8 The output ripple is highest at maximum input voltage since ∆IL increases with input voltage. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special poly- mer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important to only use types that have been surge tested for use in switching power supplies. Aluminum electrolytic capacitors have significantly higher ESR, but can be used in cost-sensitive applications provided that consideration is given to ripple current ratings and long term reliability. Ceramic capaci- tors have excellent low ESR characteristics but can have a high voltage coefficient and audible piezoelectric effects. The high Q of ceramic capacitors with trace inductance can also lead to significant ringing. 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. However, care must be taken when these capacitors are used at the input and 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 an external resistive divider according to the following equation: VV R R OUT =+ 08 1 2 1 . The resistive divider allows the VFB pin to sense a fraction of the output voltage as shown in Figure 2. Figure 2. Setting the Output Voltage LTC3416 SGND R1 3416 F02 R2 VFB VOUT Voltage Tracking The LTC3416 allows the user to program how its output voltage ramps during start-up by means of the TRACK pin. Through this pin, the output voltage can be set up to either |
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