Datasheet LTC3419 (Analog Devices) - 10

LTC3419
APPLICATIONS INFORMATION
deviations do not offer much relief. Note that capacitor However, care must be taken when ceramic capacitors are manufacturer’s ripple current ratings are often based on used at the input. When a ceramic capacitor is used at the only 2000 hours lifetime. This makes it advisable to further input and the power is supplied by a wall adapter through derate the capacitor, or choose a capacitor rated at a higher long wires, a load step at the output can induce ringing at temperature than required. Several capacitors may also be the input, VIN. At best, this ringing can couple to the output paralleled to meet the size or height requirements of the and be mistaken as loop instability. At worst, a sudden design. An additional 0.1μF to 1μF ceramic capacitor is inrush of current through the long wires can potentially also recommended on VIN for high frequency decoupling cause a voltage spike at VIN, large enough to damage the when not using an all-ceramic capacitor solution. part. For more information, see Application Note 88. When choosing the input and output ceramic capacitors,
Output Capacitor (COUT) Selection
choose the X5R or X7R dielectric formulations. These The selection of COUT is driven by the required effective dielectrics have the best temperature and voltage charac- series resistance (ESR). Typically, once the ESR requirement teristics of all the ceramics for a given value and size. for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. The output ripple
Setting the Output Voltage
ΔVOUT is determined by: The LTC3419 regulates the VFB1 and VFB2 pins to 0.6V ⎛ 1 ⎞ during regulation. Thus, the output voltage is set by a ΔVOUT ≈ ΔI E ⎜ SR L + ⎟ 8f C resistive divider according to the following formula: ⎝ O OUT ⎠ ⎛ R2⎞ where f V 0.6V 1 2 ( ) O = operating frequency, COUT = output capacitance OUT = + ⎝⎜ R1⎠⎟ and ΔI L = ripple current in the inductor. For a fi xed output voltage, the output ripple is highest at maximum input Keeping the current small (< 10μA) in these resistors voltage since ΔIL increases with input voltage. maximizes effi ciency, but making it too small may allow If tantalum capacitors are used, it is critical that the capacitors stray capacitance to cause noise problems or reduce the are surge tested for use in switching power supplies. An phase margin of the error amp loop. excellent choice is the AVX TPS series of surface mount To improve the frequency response of the main control tantalum. These are specially constructed and tested for low loop, a feedback capacitor (CF) may also be used. Great ESR so they give the lowest ESR for a given volume. Other care should be taken to route the VFB line away from noise capacitor types include Sanyo POSCAP, Kemet T510 and sources, such as the inductor or the SW line. T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specifi c recommendations. Fixed output versions of the LTC3419 (e.g. LTC3419-1) include an internal resistive divider, eliminating the need
Using Ceramic Input and Output Capacitors
for external resistors. The resistor divider is chosen such that the VFB input current is approximately 3μA. For Higher values, lower cost ceramic capacitors are now these versions the VFB pin should be connected directly becoming available in smaller case sizes. Their high to VOUT. Table 2 lists the fi xed output voltages available ripple current, high voltage rating and low ESR make for the LTC3419. them ideal for switching regulator applications. Because the LTC3419 control loop does not depend on the output
Table 2. Fixed Output Voltage Versions
capacitor’s ESR for stable operation, ceramic capacitors
PART NUMBER VOUT1 VOUT2
can be used freely to achieve very low output ripple and LTC3419 Adjustable Adjustable small circuit size. LTC3419-1 1.575V 1.8V 3419fa 10