Datasheet LTC3404 (Analog Devices) - 10

LTC3404
U U W U APPLICATIO S I FOR ATIO
inductor ripple current and consequent output voltage is highest at maximum input voltage since ΔIL increases ripple. Do not allow the core to saturate! with input voltage. For the LTC3404, the general rule for proper operation is: Kool Mμ (from Magnetics, Inc.) is a very good, low loss core material for toroids with a “soft” saturation character- COUT required ESR < 0.25Ω istic. Molypermalloy is slightly more efficient at high The choice of using a smaller output capacitance (>200kHz) switching frequencies but quite a bit more increases the output ripple voltage due to the frequency expensive. Toroids are very space efficient, especially dependent term but can be compensated for by using when you can use several layers of wire, while inductors capacitor(s) of very low ESR to maintain low ripple wound on bobbins are generally easier to surface mount. voltage. The I New designs for surface mount inductors are available TH pin compensation components can be optimized to provide stable high performance transient from Coiltronics, Coilcraft, Dale and Sumida. response regardless of the output capacitor selected.
CIN and COUT Selection
ESR is a direct function of the volume of the capacitor. Manufacturers such as Taiyo-Yuden, AVX, Kemet, Sprague In continuous mode, the source current of the top MOS- and Sanyo should be considered for high performance FET is a square wave of duty cycle VOUT/VIN. To prevent capacitors. The POSCAP solid electrolytic chip capacitor large voltage transients, a low ESR input capacitor sized available from Sanyo is an excellent choice for output bulk for the maximum RMS current must be used. The maxi- capacitors due to its low ESR/size ratio. Once the ESR mum RMS capacitor current is given by: requirement for COUT has been met, the RMS current / V [ (V −V )]1 2 rating generally far exceeds the IRIPPLE(P-P) requirement. OUT IN OUT C required I ≅ I IN RMS OMAX V When using tantalum capacitors, it is critical that they are IN surge tested for use in switching power supplies. A good This formula has a maximum at VIN = 2VOUT, where choice is the AVX TPS series of surface mount tantalum, IRMS = IOUT/2. This simple worst-case condition is com- available in case heights ranging from 2mm to 4mm. Other monly used for design because even significant deviations capacitor types include KEMET T510 and T495 series and do not offer much relief. Note that the capacitor Sprague 593D and 595D series. Consult the manufacturer manufacturer’s ripple current ratings are often based on for other specific recommendations. 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher
Output Voltage Programming
temperature than required. Several capacitors may also be The output voltage is set by a resistive divider according paralleled to meet size or height requirements in the to the following formula: design. Always consult the manufacturer if there is any question. ⎛ R2⎞ V = 0.8V 1+ OUT ⎝⎜ ⎠⎟ (2) The selection of C R1 OUT is driven by the required effective series resistance (ESR). Typically, once the ESR require- The external resistive divider is connected to the output, ment is satisfied, the capacitance is adequate for filtering. allowing remote voltage sensing as shown in Figure 3. The output ripple ΔV 0.8V OUT is determined by: ≤ VOUT ≤ 6V R2 ⎛ 1 ⎞ VFB ΔVOUT ≅ ΔI ESR L + ⎝⎜ 8fCOUT ⎠⎟ LTC3404 R1 GND where f = operating frequency, COUT = output capacitance 3404 F03 and ΔIL = ripple current in the inductor. The output ripple
Figure 3. Setting the LTC3404 Output Voltage
3404fb 10