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LT3570FE Datasheet(PDF) 11 Page - Linear Technology |
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LT3570FE Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT3570 11 3570fa to 1.2A at DC2 = 0.8. The maximum output current is a function of the chosen inductor value: IOUT2(MAX) =ILIM2 – ΔIL2 2 = 1.5 • 1– 0.25 •DC2 ()– Δ IL2 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 LT3570 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 IOUT2 is less than ΔIL2/2. Boost Inductor Selection For most applications the inductor will fall in the range of 2.2μH to 22μH. Lower values are chosen to reduce physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the power switch, which has a 1.5A current limit. Higher values also reduce input ripple voltage and reduce core loss. The following procedure is suggested as a way of choosing a more optimum inductor. Assume that the average inductor current for a boost converter is equal to the load current times VOUT1/VIN1 and decide whether or not the inductor must withstand continuous overload conditions. If average inductor cur- rent at maximum load current is 0.5A, for instance, a 0.5A inductor may not survive a continuous 1.5A overload condition. Also be aware that boost converters are not short-circuit protected, and that under short conditions, inductor current is limited only by the available current of the input supply Calculate peak inductor current at full load current to en- sure that the inductor will not saturate. Peak current can be significantly higher than output current, especially with smaller inductors and lighter loads, so don’t omit this step. Powdered iron cores are forgiving because they saturate softly, whereas ferrite cores saturate abruptly. Other core materials fall somewhere in between. The following formula assumes continuous mode operation but it errs only slightly on the high side for discontinuous mode, so it can be used for all conditions. IPEAK1 = IOUT1 •VOUT1 VIN1 + VIN1 VOUT1 –VIN1 () 2• f •L • VOUT1 Make sure that IPEAK1 is less than the switch current ILIM1. ILIM1 is at least 1.5A at low duty cycles and decreases linearly to 1.2A at DC1 = 0.8. The maximum switch current limit can be calculated by the following formula: ILIM1 = 1.5 • (1 – 0.25 • DC1) where DC1 is the duty cycle and is defined as: DC1 = 1– VIN1 VOUT1 Remember also that inductance can drop significantly with DC current and manufacturing tolerance. Consideration should also be given to the DC resistance of the inductor as this contributes directly to the efficiency losses in the overall converter. Table 1 lists several inductor vendors and types that are suitable. Buck Output Capacitor Selection For 5V and 3.3V outputs, a 10μF, 6.3V ceramic capacitor (X5R or X7R) at the output results in very low output volt- age ripple and good transient response. For lower voltages, 10μF is adequate for ripple requirements but increasing COUT will improve transient performance. Other types and values will also work; the following discusses tradeoffs in output ripple and transient performance. The output capacitor filters the inductor current to generate an output with low voltage ripple. It also stores energy in order to satisfy transient loads and stabilize the LT3570’s control loop. Because the LT3570 operates at a high frequency, minimal output capacitance is necessary. In addition, the control loop operates well with or without the presence of output capacitor series resistance (ESR). Ceramic capacitors, which achieve very low output ripple APPLICATIONS INFORMATION |
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