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LT3475EFE-1-TRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LT3475EFE-1-TRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT3475/LT3475-1 11 3475fb 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 LT3475 limits its switch cur- rent in order to protect itself and the system from overload faults. Therefore, the maximum output current that the LT3475 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 ΔIL = 1– DC () VOUT + VF () L•f () where f is the switching frequency of the LT3475 and L is the value of the inductor. The peak inductor and switch current is I SW PK () = ILPK () = IOUT + ΔIL 2 To maintain output regulation, this peak current must be less than the LT3475’s switch current limit ILIM. ILIM is at least 2.3A at low duty cycles and decreases linearly to 1.8A at DC = 0.9. The maximum output current is a function of the chosen inductor value: I OUT MAX () = ILIM – ΔIL 2 = 2.3A• 1–0.25•DC () – ΔIL 2 Choosing an inductor value so that the ripple current is small will allow a maximum output current near the switch current limit. One approach to choosing the inductor is to start with the simple rule given above, look at the available inductors, and choose one to meet cost or space goals. Then use these equations to check that the LT3475 will be able to deliver the required output current. Note again that these equations assume that the inductor current is continu- ous. Discontinuous operation occurs when IOUT is less than ΔIL/2. Input Capacitor Selection Bypass the input of the LT3475 circuit with a 4.7μF or higher ceramic capacitor of X7R or X5R type. A lower value or a less expensive Y5V type will work if there is additional bypassing provided by bulk electrolytic capaci- tors or if the input source impedance is low. The following paragraphs describe the input capacitor considerations in more detail. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input ca- pacitor is required to reduce the resulting voltage ripple at the LT3475 input 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, and it must have an adequate ripple current rat- ing. With two switchers operating at the same frequency but with different phases and duty cycles, calculating the input capacitor RMS current is not simple. However, a conservative value is the RMS input current for the channel that is delivering most power (VOUT • IOUT): CINRMS = IOUT • VOUT(VIN –VOUT ) VIN < IOUT 2 and is largest when VIN = 2VOUT (50% duty cycle). As the second, lower power channel draws input current, the input capacitor’s RMS current actually decreases as the out-of-phase current cancels the current drawn by the higher power channel. Considering that the maximum load current from a single channel is ~1.5A, RMS ripple current will always be less than 0.75A. The high frequency of the LT3475 reduces the energy storage requirements of the input capacitor, so that the capacitance required is less than 10μF. The combination of small size and low impedance (low equivalent series resistance or ESR) of ceramic capacitors makes them the preferred choice. The low ESR results in very low voltage ripple. Ceramic capacitors can handle larger magnitudes of ripple current than other capacitor types of the same value. Use X5R and X7R types. APPLICATIONS INFORMATION |
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