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LT3509 Datasheet(PDF) 15 Page - Linear Technology |
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LT3509 Datasheet(HTML) 15 Page - Linear Technology |
15 / 24 page LT3509 15 3509f APPLICATIONS INFORMATION Table 2. Recommended Inductors MANUFACTURER/ PART NUMBER VALUE (μH) ISAT (A) DCR ( Ω) HEIGHT (mm) Coilcraft LPS4018-222ML 2.2 2.8 0.07 1.7 LPS5030-332ML 3.3 2.5 0.066 2.9 LPS5030-472ML 4.7 2.5 0.083 2.9 LPS6225-682ML 6.8 2.7 0.095 2.4 LPS6225-103ML 10 2.1 0.105 2.4 Sumida CDRH4D22/HP-2R2N 2.2 3.2 0.0035 2.4 CDRH4D22/HP-3R5N 3.5 2.5 0.052 2.4 CDRH4D22/HP-4R7N 4.7 2.2 0.066 2.4 CDRH5D28/HP-6R8N 6.8 3.1 0.049 3.0 CDRH5D28/HP-8R2N 8.2 2.7 0.071 3.0 CDRH5D28R/HP-100N 10 2.45 0.074 3.0 Cooper SD52-2R2-R 2.2 2.30 0.0385 2.0 SD52-3R5-R 3.5 1.82 0.0503 2.0 SD52-4R7-R 4.7 1.64 0.0568 2.0 SD6030-5R8-R 5.8 1.8 0.045 3.0 SD7030-8R0-R 8.0 1.85 0.058 3.0 SD7030-100-R 10.0 1.7 0.065 3.0 Toko A997AS-2R2N 2.2 1.6 0.06 1.8 A997AS-3R3N 3.3 1.2 0.07 1.8 A997AS-4R7M 4.7 1.07 0.1 1.8 Würth 7447745022 2.2 3.5 0.036 2.0 7447745033 3.3 3.0 0.045 2.0 7447745047 4.7 2.4 0.057 2.0 7447745076 7.6 1.8 0.095 2.0 7447445100 10 1.6 0.12 2.0 The prior analysis is valid for continuous mode operation (IOUT > ΔILIM / 2). For details of maximum output current in discontinuous mode operation, see Linear Technology’s Application Note AN44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid subharmonic oscillations. This minimum inductance is LV V f MIN OUT F SW =+ ()• . 14 where fSW is in MHz and LMIN is in μH. If using external synchronization, calculate LMIN using the RT frequency and not the SYNC frequency. Frequency Compensation The LT3509 uses current mode control to regulate the output, which simplifies loop compensation and allows the necessary filter components to be integrated. The fixed internal compensation network has been chosen to give stable operation over a wide range of operating conditions but assumes a minimum load capacitance. The LT3509 does not depend on the ESR of the output capacitor for stability so the designer is free to use ceramic capacitors to achieve low output ripple and small PCB footprint. Figure 10 shows an equivalent circuit for the LT3509 control loop. The error amp is a transconductance amplifier with finite output impedance. The power section, consisting of the modulator, power switch and inductor is modeled as a transconductance amplifier generating an output current proportional to the voltage at the COMP-NODE. The gain of the power stage (gmp) is 1.1S. Note that the output capacitor integrates this current and that the internal capacitor integrates the error amplifier output current, resulting in two poles in the loop. In most cases, a zero is required and comes either from the output capacitor ESR |
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