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LT1934 Datasheet(PDF) 9 Page - Linear Technology |
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LT1934 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LT1934/LT1934-1 9 1934fe the average inductor current equals the load current, the maximum load current is: IOUT(MAX) = IPK – ΔIL/2 where IPK is the peak inductor current and ΔIL is the peak-to-peak ripple current in the inductor. The ripple current is determined by the off time, tOFF = 1.8μs, and the inductor value: ΔIL = (VOUT + VD) • tOFF/L IPK is nominally equal to ILIM. However, there is a slight delay in the control circuitry that results in a higher peak current and a more accurate value is: IPK = ILIM + 150ns • (VIN – VOUT)/L These expressions are combined to give the maximum load current that the LT1934 will deliver: IOUT(MAX) = 350mA + 150ns • (VIN – VOUT)/L – 1.8μs • (VOUT + VD)/2L (LT1934) IOUT(MAX) = 90mA + 150ns • (VIN – VOUT)/L – 1.8μs • (VOUT + VD)/2L (LT1934-1) The minimum current limit is used here to be conservative. The third term is generally larger than the second term, so that increasing the inductor value results in a higher output current. This equation can be used to evaluate a chosen inductor or it can be used to choose L for a given maximum load current. The simple, single equation rule given above for choosing L was found by setting ΔIL = ILIM/2.5. This results in IOUT(MAX) ~0.8ILIM (ignoring the delay term). Note that this analysis assumes that the inductor current is continuous, which is true if the ripple current is less than the peak current or ΔIL < IPK. The inductor must carry the peak current without satu- rating excessively. When an inductor carries too much current, its core material can no longer generate ad- ditional magnetic flux (it saturates) and the inductance drops, sometimes very rapidly with increasing current. This condition allows the inductor current to increase at a very high rate, leading to high ripple current and decreased overload protection. Inductor vendors provide current ratings for power induc- tors. These are based on either the saturation current or on the RMS current that the inductor can carry without dissipating too much power. In some cases it is not clear which of these two determine the current rating. Some data sheets are more thorough and show two current ratings, one for saturation and one for dissipation. For LT1934 ap- plications, the RMS current rating should be higher than the load current, while the saturation current should be higher than the peak inductor current calculated above. Input Capacitor Step-down regulators draw current from the input sup- ply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT1934 and to force this switching current into a tight local loop, minimizing EMI. The input capacitor must have low impedance at the switching frequency to do this effectively. A 2.2μF ceramic capacitor (1μF for the LT1934-1) satisfies these requirements. If the input source impedance is high, a larger value ca- pacitor may be required to keep input ripple low. In this case, an electrolytic of 10μF or more in parallel with a 1μF ceramic is a good combination. Be aware that the input Table 2. Inductor Vendors VENDOR PHONE URL PART SERIES COMMENTS Murata (404) 426-1300 www.murata.com LQH3C Small, Low Cost, 2mm Height Sumida (847) 956-0666 www.sumida.com CR43 CDRH4D28 CDRH5D28 Coilcraft (847) 639-6400 www.coilcraft.com DO1607C DO1608C DT1608C Würth Electronics (866) 362-6673 www.we-online.com WE-PD1, 2, 3, 4 APPLICATIONS INFORMATION |
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Similar Description - LT1934_15 |
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