Datasheet LTC3416 (Analog Devices) - 9

LTC3416
U U W U APPLICATIO S I FOR ATIO
prevent large voltage transients from occurring, a low ESR high voltage coefficient and audible piezoelectric effects. input capacitor sized for the maximum RMS current The high Q of ceramic capacitors with trace inductance should be used. The maximum RMS current is given by: can also lead to significant ringing. V V
Using Ceramic Input and Output Capacitors
I = I OUT IN RMS OUT MAX ( ) – 1 V V IN OUT Higher values, lower cost ceramic capacitors are now This formula has a maximum at V becoming available in smaller case sizes. Their high ripple IN = 2VOUT, where IRMS = I current, high voltage rating and low ESR make them ideal OUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not for switching regulator applications. However, care must offer much relief. Note that ripple current ratings from be taken when these capacitors are used at the input and capacitor manufacturers are often based on only 2000 output. When a ceramic capacitor is used at the input and hours of life which makes it advisable to further derate the the power is supplied by a wall adapter through long wires, capacitor, or choose a capacitor rated at a higher tempera- a load step at the output can induce ringing at the input, ture than required. Several capacitors may also be paral- VIN. At best, this ringing can couple to the output and be leled to meet size or height requirements in the design. For mistaken as loop instability. At worst, a sudden inrush of low input voltage applications, sufficient bulk input ca- current through the long wires can potentially cause a pacitance is needed to minimize transient effects during voltage spike at VIN large enough to damage the part. output load changes. When choosing the input and output ceramic capacitors, The selection of C choose the X5R or X7R dielectric formulations. These OUT is determined by the effective series resistance (ESR) that is required to minimize voltage dielectrics have the best temperature and voltage charac- ripple and load step transients as well as the amount of teristics of all the ceramics for a given value and size. bulk capacitance that is necessary to ensure that the control loop is stable. Loop stability can be checked by
Output Voltage Programming
viewing the load transient response as described in a later The output voltage is set by an external resistive divider section. The output ripple, ∆VOUT, is determined by: according to the following equation: ∆ 1 ⎛ 2⎞ V R OUT ≤ ∆ ⎛ I ESR L + ⎞ ⎝⎜ V 0.8V 1 8fC OUT = + OUT ⎠ ⎟ ⎝⎜ R ⎠⎟ 1 The output ripple is highest at maximum input voltage The resistive divider allows the VFB pin to sense a fraction since ∆IL increases with input voltage. Multiple capacitors of the output voltage as shown in Figure 2. placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special poly- VOUT mer, aluminum electrolytic and ceramic capacitors are all R2 available in surface mount packages. Special polymer VFB capacitors offer very low ESR but have lower capacitance LTC3416 R1 density than other types. Tantalum capacitors have the SGND highest capacitance density but it is important to only use 3416 F02 types that have been surge tested for use in switching
Figure 2. Setting the Output Voltage
power supplies. Aluminum electrolytic capacitors have significantly higher ESR, but can be used in cost-sensitive
Voltage Tracking
applications provided that consideration is given to ripple The LTC3416 allows the user to program how its output current ratings and long term reliability. Ceramic capaci- voltage ramps during start-up by means of the TRACK pin. tors have excellent low ESR characteristics but can have a Through this pin, the output voltage can be set up to either 3416fa 9