Datasheet LTC3405A-1.375 (Analog Devices) - 8

LTC3405A-1.375
U U W U APPLICATIO S I FOR ATIO
Table 1 shows some typical surface mount inductors that ⎛ 1 ⎞ work well in LTC3405A-1.375 applications. ∆VOUT ≅ ∆I ESR L + ⎜ ⎟ ⎝ 8fCOUT ⎠
Table 1. Representative Surface Mount Inductors MAX DC
where f = operating frequency, COUT = output capacitance
MANUFACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
and ∆IL = ripple current in the inductor. For a fixed output Taiyo Yuden LB2016T2R2M 2.2µH 315mA 0.13Ω 1.6mm voltage, the output ripple is highest at maximum input LB2012T2R2M 2.2µH 240mA 0.23Ω 1.25mm LB2016T3R3M 3.3µH 280mA 0.2Ω 1.6mm voltage since ∆IL increases with input voltage. Panasonic ELT5KT4R7M 4.7µH 950mA 0.2Ω 1.2mm Aluminum electrolytic and dry tantalum capacitors are Murata LQH32CN2R2M33 4.7µH 450mA 0.2Ω 2mm both available in surface mount configurations. In the case Taiyo Yuden LB2016T4R7M 4.7µH 210mA 0.25Ω 1.6mm of tantalum, it is critical that the capacitors are surge tested Panasonic ELT5KT6R8M 6.8µH 760mA 0.3Ω 1.2mm for use in switching power supplies. An excellent choice is Panasonic ELT5KT100M 10µH 680mA 0.36Ω 1.2mm the AVX TPS series of surface mount tantalum. These are Sumida CMD4D116R8MC 6.8µH 620mA 0.23Ω 1.2mm specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP, Kemet T510 and T495 series, and
CIN and COUT Selection
Sprague 593D and 595D series. Consult the manufacturer In continuous mode, the source current of the top MOSFET for other specific recommendations. is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the
Using Ceramic Input and Output Capacitors
maximum RMS current must be used. The maximum Higher values, lower cost ceramic capacitors are now RMS capacitor current is given by: becoming available in smaller case sizes. Their high ripple / current, high voltage rating and low ESR make them ideal V V − V OUT [ ( IN OUT)]12 for switching regulator applications. Because the C required I I IN RMS ≅ OMAX V LTC3405A-1.375’s control loop does not depend on the IN output capacitor’s ESR for stable operation, ceramic ca- This formula has a maximum at VIN = 2VOUT, where pacitors can be used freely to achieve very low output IRMS = IOUT/2. This simple worst-case condition is com- ripple and small circuit size. monly used for design because even significant deviations Care must be taken when ceramic capacitors are used at do not offer much relief. Note that the capacitor the input and the output. When a ceramic capacitor is used manufacturer’s ripple current ratings are often based on at the input and the power is supplied by a wall adapter 2000 hours of life. This makes it advisable to further derate through long wires, a load step at the output can induce the capacitor, or choose a capacitor rated at a higher ringing at the input, VIN. At best, this ringing can couple to temperature than required. Always consult the manufac- the output and be mistaken as loop instability. At worst, a turer if there is any question. sudden inrush of current through the long wires can The selection of COUT is driven by the required effective potentially cause a voltage spike at VIN, large enough to series resistance (ESR). Typically, once the ESR require- damage the part. ment for COUT has been met, the RMS current rating When choosing the input and output ceramic capacitors, generally far exceeds the IRIPPLE(P-P) requirement. The choose the X5R or X7R dielectric formulations. These output ripple ∆VOUT is determined by: dielectrics have the best temperature and voltage charac- teristics of all the ceramics for a given value and size. 3405a1375f 8