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LT3570 Datasheet(PDF) 10 Page - Linear Technology |
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LT3570 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LT3570 10 3570fa APPLICATIONS INFORMATION FB Resistor Network The output voltage is programmed with a resistor divider (refer to the Block Diagram) between the output and the FB pin. Choose the resistors according to: R1 =R2 VOUT 788mV –1 ⎛ ⎝⎜ ⎞ ⎠⎟ Buck Inductor Selection and Maximum Output Current A good first choice for the inductor value is L = VOUT2 + VF 0.75 • f for SW2 where VF is the voltage drop of the catch diode (~0.4V) and f is the switching frequency. With this inductance value or greater, the maximum load current will be 1A, independent of input voltage. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be at least 30% higher. For highest efficiency, the series resistance (DCR) should be less than 0.1Ω. Table 1 lists several vendors and types that are suitable. Table 1. Inductors PART NUMBER VALUE (μH) ISAT (A) DCR ( Ω) HEIGHT (mm) Sumida CDRH4D28-3R3 3.3 1.57 0.049 3.0 CDRH4D28-4R7 4.7 1.32 0.072 3.0 CDC5D23-2R2 2.2 2.50 0.03 2.5 CR43-3R3 3.3 1.44 0.086 3.5 CDRH5D28-100 10 1.3 0.048 3.0 Coilcraft DO1608C-332 3.3 2.00 0.080 2.9 DO1608C-472 4.7 1.50 0.090 2.9 MOS6020-332 3.3 1.8 0.046 2.0 D03314-103 10 0.8 0.520 1.4 D03314-222 2.2 1.6 0.200 1.4 Toko (D62F)847FY-2R4M 2.4 2.5 0.037 2.7 (D73LF)817FY-2R2M 2.2 2.7 0.03 3.0 Coiltronics TP3-4R7 4.7 1.5 0.181 2.2 TP1-2R2 2.2 1.3 0.188 1.8 TP4-100 10 1.5 0.146 3.0 The optimum inductor for a given application may differ from the one indicated by this simple design guide. A larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. If your load is lower than the maximum load current, then you can relax the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor or one with a lower DCR resulting in higher efficiency. Be aware that if the inductance differs from the simple rule above, then the maximum load current will depend on input voltage. In addition, low inductance may result in discontinuous mode operation, which further reduces maximum load current. For details of maximum output current and discontinuous mode operation, see Linear Technology’s Application Note 44. Finally, for duty cycles greater than 50% (VOUT2/VIN2 > 0.5) a minimum inductance is required to avoid subharmonic oscillations, see Application Note 19. The current in the inductor is a triangle wave with an average value equal to the load current. The peak switch current is equal to the output current plus half the peak-to-peak inductor ripple current. The LT3570 limits its switch cur- rent in order to protect itself and the system from overload faults. Therefore, the maximum output current that the LT3570 will deliver depends on the switch current limit, the inductor value and the input and output voltages. When the switch is off, the potential across the inductor is the output voltage plus the catch diode drop. This gives the peak-to-peak ripple current in the inductor: ΔIL2 = 1– DC2 () V OUT2 + VF () L•f where DC2 is the duty cycle and is defined as: DC2 = VOUT2 VIN2 The peak inductor and switch current is: ISWPK2 =ILPK2 =IOUT2 + ΔIL2 2 To maintain output regulation, this peak current must be less than the LT3570’s switch current limit ILIM2. ILIM2 is at least 1.5A at low duty cycles and decreases linearly |
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