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LTC3403EDD Datasheet(PDF) 10 Page - Linear Technology |
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LTC3403EDD Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page 10 LTC3403 3403f Table 1. Representative Surface Mount Inductors Part Value DCR MAX DC Size Number ( µH) ( ΩMAX) Current (A) WxLxH (mm3) Sumida 1.5 0.068 0.90 3.2 x 3.2 x 1.2 CDRH2D11 2.2 0.098 0.78 3.3 0.123 0.60 Sumida 2.2 0.041 0.85 3.2 x 3.2 x 2.0 CDRH2D18/LD 3.3 0.054 0.75 4.7 0.078 0.63 Sumida 2.2 0.116 0.95 3.5 x 4.1 x 0.8 CMD4D06 3.3 0.174 0.77 4.7 0.216 0.75 Murata 1.0 0.060 1.00 2.5 x 3.2 x 2.0 LQH32C 2.2 0.097 0.79 4.7 0.150 0.65 Taiyo Yuden 1.0 0.080 0.78 1.8 x 2.5 x 1.8 LQLBC2518 1.5 0.110 0.66 2.2 0.130 0.60 Toko 2.2 0.14 1.14 4.6 x 4.6 x 1.2 D412F 3.3 0.20 0.90 4.7 0.22 0.80 CIN and COUT Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: C required I I VV V V IN RMS OMAX OUT IN OUT IN ≅ [( – )] / 12 This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that the capacitor manufacturer’s ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Always consult the manufacturer if there is any question. The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR require- ment for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. The output ripple VOUT is determined by: ∆≅ ∆ + V I ESR fC OUT L OUT 1 8 where f = operating frequency, COUT = output capacitance and IL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since IL increases with input voltage. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. In the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalum. These are 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 Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations. The bulk capacitance values in Figure 1(a) (CIN = 10µF, COUT = 4.7µF) are tailored to mobile phone applications, in which the output voltage is expected to slew quickly according to the needs of the power amplifier. Holding the output capacitor to 4.7 µF facilitates rapid charging and discharging. When the output voltage descends quickly in forced continuous mode, the LTC3403 will actually pull current from the output until the command from VREF is satisfied. On alternate half cyles, this current actually exits the VIN terminal, potentially causing a rise in VIN and forcing current into the battery. To prevent deterioration of the battery, use sufficient bulk capacitance with low ESR; at least 10 µF is recommended. APPLICATIO S I FOR ATIO |
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