Datasheet LTC3410B (Analog Devices) - 9

LTC3410B
U U W U APPLICATIO S I FOR ATIO
This formula has a maximum at V
Using Ceramic Input and Output Capacitors
IN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is com- Higher values, lower cost ceramic capacitors are now monly used for design because even significant deviations becoming available in smaller case sizes. Their high ripple do not offer much relief. Note that the capacitor current, high voltage rating and low ESR make them ideal manufacturer’s ripple current ratings are often based on for switching regulator applications. Because the 2000 hours of life. This makes it advisable to further derate LTC3410B’s control loop does not depend on the output the capacitor, or choose a capacitor rated at a higher capacitor’s ESR for stable operation, ceramic capacitors temperature than required. Always consult the manufac- can be used freely to achieve very low output ripple and turer if there is any question. small circuit size. The selection of COUT is driven by the required effective However, care must be taken when ceramic capacitors are series resistance (ESR). Typically, once the ESR require- used at the input and the output. When a ceramic capacitor ment for COUT has been met, the RMS current rating is used at the input and the power is supplied by a wall generally far exceeds the IRIPPLE(P-P) requirement. The adapter through long wires, a load step at the output can output ripple ∆VOUT is determined by: induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At ∆ 1 VOUT ≅ ∆ ⎛ I ESR L + ⎞ worst, a sudden inrush of current through the long wires ⎝⎜ fC 8 OUT ⎠⎟ can potentially cause a voltage spike at VIN, large enough to damage the part. where f = operating frequency, COUT = output capacitance and ∆I When choosing the input and output ceramic capacitors, L = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input choose the X5R or X7R dielectric formulations. These voltage since ∆I dielectrics have the best temperature and voltage charac- L increases with input voltage. teristics of all the ceramics for a given value and size. If tantalum capacitors are used, it is critical that the capacitors are surge tested for use in switching power
Output Voltage Programming (LTC3410B Only)
supplies. An excellent choice is the AVX TPS series of The output voltage is set by a resistive divider according surface mount tantalum. These are specially constructed to the following formula: and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo ⎛ R2⎞ V 0. V POSCAP, Kemet T510 and T495 series, and Sprague 593D OUT = 8 1+ ⎝⎜ (2) R ⎠⎟ 1 and 595D series. Consult the manufacturer for other The external resistive divider is connected to the output, specific recommendations. allowing remote voltage sensing as shown in Figure 2. 0.8V ≤ VOUT ≤ 5.5V R2 VFB LTC3410B R1 GND 3410 F02
Figure 2. Setting the LTC3410B Output Voltage
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