Datasheet LTC3549 (Analog Devices) - 10

LTC3549
U U W U APPLICATIO S I FOR ATIO
tions do not offer much relief. Note that the capacitor value, lower cost ceramic capacitors are now available in manufacturer’s ripple current ratings are often based smaller case sizes. Their high ripple current, high voltage on 2000 hours of life. This makes it advisable to further rating and low ESR make them ideal for switching regulator derate the capacitor, or choose a capacitor rated at a applications. Because the LTC3549’s control loop does not higher temperature than required. Always consult the depend on the output capacitor’s ESR for stable operation, manufacturer if there is any question. The selection of ceramic capacitors can be used to achieve very low output COUT is driven by the required effective series resistance ripple and small circuit size. (ESR). Typically, once the ESR requirement for COUT has However, care must be taken when these capacitors are been met, the RMS current rating generally far exceeds used at the input and the output. When a ceramic capacitor the IRIPPLE(P-P) requirement. The output ripple ΔVOUT is used at the input and the power is supplied by a wall is determined by: adapter through long wires, a load step at the output can ∆ 1 V induce ringing at the input, VIN. At best, this ringing can OUT = ∆ ⎛ I ESR L + ⎞ ⎝⎜ 8 • f • COUT ⎠⎟ couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires where f = operating frequency, COUT = output capacitance can potentially cause a voltage spike at VIN, large enough and ΔIL = ripple current in the inductor. For a fi xed output to damage the part. voltage, the output ripple is highest at maximum input When choosing the input and output ceramic capacitors, voltage since ΔIL increases with input voltage. Aluminum choose the X5R or X7R dielectric formulations. These electrolytic and dry tantalum capacitors are both available dielectrics have the best temperature and voltage charac- in surface mount confi gurations. In the case of tantalum, teristics of all the ceramics for a given value and size. it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the
Output Voltage Programming
AVX TPS series of surface mount tantalum. These are specially constructed and tested for low ESR so they give The output voltage is set by a resistive divider according the lowest ESR for a given volume. Other capacitor types to the following formula: include Sanyo POSCAP, Kemet T510 and T495 series, and ⎛ R1⎞ V 0. V 611 1 Sprague 593D and 595D series. Consult the manufacturer OUT = + ⎝⎜ R2⎠⎟ (2) for other specifi c recommendations. The external resistive divider is connected to the output,
Using Ceramic Input and Output Capacitors
allowing remote voltage sensing as shown in Figure 1 The LTC3549 typically will require an output capacitor Table 2 gives 1% resistor values for selected output in the 4.7µF to 10µF range for optimum stability. Higher voltages. VOUT
Table 2. Resistor Values for Selected Output Voltages V
R1
OUT R1 R2
LTC3549 0.85V 53.6k 137k VFB 1.2V 133k 137k R2 1.5V 200k 137k GND 1.8V 267k 137k 3549 F01
Figure 1. Setting the LTC3549 Output Voltage
3549f 10