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LT1765-2.5 Datasheet(PDF) 9 Page - Linear Technology |
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LT1765-2.5 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LT1765/LT1765-1.8/LT1765-2.5/ LT1765-3.3/LT1765-5 9 sn1765 1765fas INDUCTOR CHOICE AND MAXIMUM OUTPUT CURRENT Maximum output current for an LT1765 buck converter is equal to the maximum switch rating (IP) minus one half peak to peak inductor ripple current. The LT1765 maintains a constant switch current rating at all duty cycles. (Patent Pending) For most applications, the output inductor will be in the 1 µH to 10µH range. Lower values are chosen to reduce the physical size of the inductor, higher values allow higher output currents due to reduced peak to peak ripple current. The following formula gives maximum output current for continuous mode operation, implying that the peak to peak ripple (2x the term on the right) is less than the maximum switch current. Figure 3. Output Ripple Voltage Waveform VOUT USING 47µF, 0.1Ω TANTALUM CAPACITOR (10mV/DIV) VSW (5V/DIV) VOUT USING 2.2µF CERAMIC CAPACITOR (10mV/DIV) 0.2 µs/DIV 1765 F03 APPLICATIO S I FOR ATIO IOUT MAX () = Continuous Mode I VV V Lf V P OUT IN OUT IN – ()( – ) ()( )( ) 2 For VIN = 8V, VOUT = 5V and L = 3.3µH, I A OUT MAX () .• . • .. =− () − () ()( )() =− = − 3 58 5 2 3 3 10 1 25 10 8 3 0 23 2 77 66 Note that the worst case (minimum output current avail- able) condition is at the maximum input voltage. For the same circuit at 15V, maximum output current would be only 2.6A. Inductor Selection The output inductor should have a saturation current rating greater than the peak inductor current set by the current comparator of the LT1765. The peak inductor current will depend on the output current, input and output voltages and the inductor value: II VV V Lf V PEAK OUT OUT IN OUT IN =+ − () ()( )( ) 2 VIN = Maximum input voltage f = Switching frequency, 1.25MHz If an inductor with a peak current lower than the maximum switch current of the LT1765 is chosen a soft-start circuit in Figure 10 should be used. Also, short-circuit conditions should not be allowed because the inductor may saturate resulting in excessive power dissipation. Also, consideration should be given to the resistance of the inductor. Inductor conduction loses are directly pro- portional to the DC resistance of inductor. Sometime, the manufacturers will also provide maximum current rating based on the allowable losses in the inductor. Care should be taken, however. At high input voltages and low DCR, excessive switch current could flow during shorted output condition. Suitable inductors are available from Coilcraft, Coiltronics, Dale, Sumida, Toko, Murata, Panasonic and other manufacturers. Table 2. Surface Mount Solid Tantalum Capacitor ESR and Ripple Current E Case Size ESR (Max, Ω) Ripple Current (A) AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 AVX TAJ 0.7 to 0.9 0.4 D Case Size AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 C Case Size AVX TPS 0.2 (typ) 0.5 (typ) Figure 3 shows a comparison of output ripple for a ceramic and tantalum capacitor at 200mA ripple current. |
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