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LT1307 Datasheet(PDF) 10 Page - Linear Technology |
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LT1307 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LT1307/LT1307B 1307fa To eliminate the low frequency noise of Figure 6, the LT1307 can be replaced with the LT1307B. Figure 9 details the spectral noise at the output of Figure 1’s circuit using an LT1307B at 5mA load. Although spectral energy is present at 333kHz due to alternate pulse skipping, all Burst Mode operation spectral components are gone. Alternate pulse skipping can be eliminated by increasing inductance. FREQUENCY COMPENSATION Obtaining proper values for the frequency compensation network is largely an empirical, iterative procedure, since variations in input and output voltage, topology, capacitor value and ESR, and inductance make a simple formula elusive. As an example, consider the case of a 1.25V to 3.3V boost converter supplying 50mA. To determine optimum compensation, the circuit is built and a transient load is applied to the circuit. Figure 10 shows the setup. away from 455kHz. Figure 8 shows the noise spectrum of the converter with the load increased to 20mA. The LT1307 shifts out of Burst Mode operation, eliminating low frequency ripple. Spectral energy is present only at the switching fundamental and its harmonics. Noise voltage measures – 5dBmVRMS or 560µVRMS at the 575kHz switching frequency, and is below – 60dBmVRMS for all other frequencies in the range. By combining Burst Mode with fixed frequency operation, the LT1307 keeps noise away from 455kHz. VIN VOUT FB 1307 • F10 GND SW 1 µF 10 µH MBR0520L C R 590k 1M 50 Ω 66 Ω 3300 Ω 1.25V VC 10 µF* *CERAMIC SHDN LT1307 Figure 10. Boost Converter with Simulated Load Figure 11a details transient response without compensa- tion components. Although the output ripple voltage at a 1mA load is low, allowing the error amplifier to operate wideband results in excessive ripple at a 50mA load. Some kind of loop stabilizing network is obviously required. A 100k/22nF series RC is connected to the VC pin, resulting in the response pictured in Figure 11b. The output settles in about 7ms to 8ms. This may be acceptable, but we can do better. Reducing C to 2nF gives Figure 11c’s response. This is clearly in the right direction. After another order of magnitude reduction, Figure 11d’s response shows some FREQUENCY (kHz) 205 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 LT1307 • F09 455 705 Figure 9. LT1307B at 5mA Load Shows No Audio Components or Sidebands About Switching Frequency, 333kHz Fundamental Amplitude is –10dBmV, or 316 µVRMS FREQUENCY (kHz) 255 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 455 1307 F08 655 RBW = 100Hz Figure 8. With Converter Delivering 20mA, Low Frequency Sidebands Disappear. Noise is Present Only at the 575kHz Switching Frequency APPLICATIO S I FOR ATIO |
Similar Part No. - LT1307_15 |
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Similar Description - LT1307_15 |
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