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LTC3558 28 3558f Table 4. Recommended Inductors for the Buck-Boost Switching Regulator. INDUCTOR TYPE L (μH) MAX IDC (A) MAX DCR (m Ω) SIZE IN mm (L × W × H) MANUFACTURER DB3018C D312C DE2812C DE2812C 2.4 2.2 2 2.7 1.31 1.14 1.4 1.2 80 140 81 87 3.8 × 3.8 × 1.4 3.6 × 3.6 × 1.2 3 × 3.2 × 1.2 3 × 3.2 × 1.2 Toko www.toko.com CDRH3D16 2.2 1.2 72 4 × 4 × 1.8 Sumida www.sumida.com SD12 2.2 1.8 74 5.2 × 5.2 × 1.2 Cooper www.cooperet.com *Typical DCR APPLICATIONS INFORMATION Input Current Limit The input current limit comparator will shut the input PMOS switch off once current exceeds 700mA typical. Before the switch current limit, the average current limit amp (620mA typical) will source current into the feedback pin to drop the output voltage. The input current limit also protects against a short-circuit condition at the VOUT2 pin. Reverse Current Limit The reverse current limit comparator will shut the output PMOS switch off once current returning from the output exceeds 450mA typical. Output Overvoltage Protection If the feedback node were inadvertently shorted to ground, then the output would increase indefinitely with the maxi- mum current that could be sourced from the input supply. The buck-boost regulator protects against this by shutting off the input PMOS if the output voltage exceeds a 5.75V maximum. Buck-Boost Regulator Soft-Start Operation Soft-start is accomplished by gradually increasing the reference voltage over a 500μs typical period. A soft- start cycle occurs whenever the buck-boost is enabled, or after a fault condition has occurred (thermal shutdown or UVLO). A soft-start cycle is not triggered by changing operating modes. This allows seamless output operation when transitioning between Burst Mode operation and PWM mode operation. Buck-Boost Switching Regulator Inductor Selection The buck-boost switching regulator is designed to work with inductors in the range of 1μH to 5μH. For most applications, a 2.2μH inductor will suffice. Larger value inductors reduce ripple current which improves output ripple voltage. Lower value inductors result in higher ripple current and improved transient response time. To maximize efficiency, choose an inductor with a low DC resistance and a DC current rating at least 1.5 times larger than the maximum load current to ensure that the inductor does not saturate during normal operation. If output short-circuit is a possible condition, the inductor current should be rated to handle up to the peak current specified for the buck-boost regulator. The inductor value also affects Burst Mode operation. Lower inductor values will cause Burst Mode switching frequencies to increase. Different core materials and shapes will change the size/cur- rent and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and do not radiate much energy, but cost more than powdered iron core inductors with similar electrical characteristics. Inductors that are very thin or have a very small volume typically have much higher core and DCR losses and will not give the best efficiency. Table 4 shows some inductors that work well with the buck-boost regulator. These inductors offer a good com- promise in current rating, DCR and physical size. Consult each manufacturer for detailed information on their entire selection of inductors. |
