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LT3508IUF-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LT3508IUF-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT3508 11 3508fb The optimum inductor for a given application may differ from the one indicated by this simple design guide. A larger value inductor provides a higher maximum load current, and reduces 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 cur- rent. 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% (VOUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillations: LV V kHz f MIN OUT F =+ ()• 800 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 LT3508 limits its switch cur- rent in order to protect itself and the system from overload faults. Therefore, the maximum output current that the LT3508 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: ΔI DC V V Lf L OUT F = () + () 1– • where f is the switching frequency of the LT3508 and L is the value of the inductor. The peak inductor and switch current is: II I I SW PK L PK OUT L () () == + Δ 2 To maintain output regulation, this peak current must be less than the LT3508’s switch current limit ILIM. ILIM is at least 2A for at low duty cycles and decreases linearly to 1.55A at DC = 90%. The maximum output current is a function of the chosen inductor value: II I ADC I OUT MAX LIM LL () –• – . • – == () ΔΔ 2 21 0 25 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 LT3508 will be able to deliver the required output current. Note again that these equations assume that the inductor current is continuous. Discontinu- ous operation occurs when IOUT is less than ΔIL/2. Input Capacitor Selection Bypass the VIN pins of the LT3508 circuit with a ceramic capacitor of X7R or X5R type. For switching frequencies above 500kHz, use a 4.7μF capacitor or greater. For switch- ing frequencies below 500kHz, use a 10μF or higher capaci- tor. If the VIN pins are tied together only a single capacitor is necessary. If the VIN pins are separated, each pin will need its own bypass. 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 LT3508 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 rating. 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 times IOUT): CI VV V V I IN RMS OUT OUT IN OUT IN OUT () • – = () < 2 and is largest when VIN = 2VOUT (50% duty cycle). As the second, lower power channel draws input current, APPLICATIONS INFORMATION |
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