Datasheet LTC3448 (Analog Devices) - 10

LTC3448
U U W U APPLICATIO S I FOR ATIO Inductor Core Selection
temperature than required. Always consult the manufac- turer if there is any question. Different core materials and shapes will change the size/ current and price/current relationship of an inductor. The selection of COUT is driven by the required effective Toroid or shielded pot cores in ferrite or permalloy mate- series resistance (ESR). Typically, once the ESR require- rials are small and don’t radiate much energy, but gener- ment for COUT has been met, the RMS current rating ally cost more than powdered iron core inductors with generally far exceeds the IRIPPLE(P-P) requirement. In any similar electrical characteristics. The choice of which style case, if LDO mode is enabled, the value of COUT must have inductor to use often depends more on the price vs size a minimum value of 2µF to ensure loop stability. The requirements and any radiated field/EMI requirements output ripple ∆VOUT is determined by: than on what the LTC3448 requires to operate. Table 1 shows some typical surface mount inductors that work ⎛ 1 ⎞ ∆VOUT ≅ ∆I ESR L + well in LTC3448 applications. ⎝⎜ fC 8 OUT ⎠⎟
Table 1. Representative Surface Mount Inductors
where f = operating frequency, COUT = output capacitance
PART VALUE DCR MAX DC SIZE
and ∆IL = ripple current in the inductor. For a fixed output
NUMBER (
µ
H) (
Ω
MAX) CURRENT (A) W
×
L
×
H (mm3)
voltage, the output ripple is highest at maximum input Sumida 1.5 0.043 1.55 3.8 × 3.8 × 1.8 CDRH3D16 2.2 0.075 1.20 voltage since ∆IL increases with input voltage. 3.3 0.110 1.10 Aluminum electrolytic and dry tantalum capacitors are 4.7 0.162 0.90 both available in surface mount configurations. In the case Sumida 2.2 0.116 0.950 3.5 × 4.3 × 0.8 CMD4D06 3.3 0.174 0.770 of tantalum, it is critical that the capacitors are surge tested 4.7 0.216 0.750 for use in switching power supplies. An excellent choice is Coilcraft 2.2 0.104 1.8 2.5 × 3.2 × 2.0 the AVX TPS series of surface mount tantalum. These are ME3220 3.3 0.138 1.3 specially constructed and tested for low ESR so they give 4.7 0.190 1.2 the lowest ESR for a given volume. Other capacitor types Murata 1.0 0.060 1.00 2.5 × 3.2 × 2.0 include Sanyo POSCAP, Kemet T510 and T495 series, and LQH3C 2.2 0.097 0.79 4.7 0.150 0.65 Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations.
CIN and COUT Selection Using Ceramic Input and Output Capacitors
In continuous mode, the source current of the top MOS- FET is a square wave of duty cycle V Higher values, lower cost ceramic capacitors are now OUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized becoming available in smaller case sizes. Their high ripple for the maximum RMS current must be used. The maxi- current, high voltage rating and low ESR make them ideal mum RMS capacitor current is given by: for switching regulator applications. Because the LTC3448’s control loop does not depend on the output / capacitor’s ESR for stable operation, ceramic capacitors V V [ ( −V )]12 OUT IN OUT can be used
freely
to achieve very low output ripple and C required I I IN RMS ≅ OMAX V small circuit size. IN However, care must be taken when ceramic capacitors are This formula has a maximum at VIN = 2VOUT, where used at the input and the output. When a ceramic capacitor IRMS = IOUT/2. This simple worst-case condition is com- is used at the input and the power is supplied by a wall monly used for design. Note that the capacitor adapter through long wires, a load step at the output can manufacturer’s ripple current ratings are often based on induce ringing at the input, V 2000 hours of life. This makes it advisable to further derate IN. At best, this ringing can couple to the output and be mistaken as loop instability. At the capacitor, or choose a capacitor rated at a higher 3448f 10