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LT3508HFE-PBF 数据表(PDF) 14 Page - Linear Technology |
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LT3508HFE-PBF 数据表(HTML) 14 Page - Linear Technology |
14 / 24 page LT3508 14 3508fb The minimum operating voltage of an LT3508 application is limited by the undervoltage lockout (≈3.7V) and by the maximum duty cycle. The boost circuit also limits the minimum input voltage for proper start-up. If the input voltage ramps slowly, or the LT3508 turns on when the output is already in regulation, the boost capacitor may not be fully charged. Because the boost capacitor charges with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. The minimum load current generally goes to zero once the circuit has started. Figure 5 shows a plot of minimum load to start and to run as a function of input voltage. Even without an output load current, in many cases the discharged output capacitor will present a load to the switcher that will allow it to start. The plots show the worst case, where VIN is ramping very slowly. Frequency Compensation The LT3508 uses current mode control to regulate the output. This simplifies loop compensation. In particular, the LT3508 does not require the ESR of the output capacitor for stability, so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. Frequency compensation is provided by the components tied to the VCpin,asshowninFigure1.Generallyacapacitor (CC) and a resistor (RC) in series to ground are used. In addition, there may be a lower value capacitor in parallel. This capacitor (CF) is not part of the loop compensation but is used to filter noise at the switching frequency, and is required only if a phase-lead capacitor is used or if the output capacitor has high ESR. Loop compensation determines the stability and transient performance. Designing the compensation network is a bit complicated and the best values depend on the application and in particular the type of output capacitor. A practical approach is to start with one of the circuits in this data sheet that is similar to your application and tune the com- pensation network to optimize the performance. Stability should then be checked across all operating conditions, including load current, input voltage and temperature. The LT1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the stability using a transient load. Figure 6 shows an equivalent circuit for the LT3508 control loop. The error amplifier 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 VC pin. Note that the output capacitor integrates this current, and that the capacitor on the VC pin (CC) integrates the error amplifier output current, resulting in two poles in the loop. In most cases a zero is required and comes from either the output capacitor ESR or from a resistor RC in series with CC. This simple model works well as long as the value of the inductor is not too high and the loop crossover frequency APPLICATIONS INFORMATION Figure 5. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit LOAD CURRENT (mA) 1 5.0 5.5 6.5 1000 3508 F05a 4.5 4.0 10 100 10000 3.5 3.0 6.0 TA = 25°C VOUT = 3.3V TO START TO RUN LOAD CURRENT (mA) 1 6 7 10000 3508 G05b 5 4 10 100 1000 9 8 TA = 25°C VOUT = 5V TO START TO RUN Minimum Input Voltage, VOUT = 3.3V Minimum Input Voltage, VOUT = 5V |
类似零件编号 - LT3508HFE-PBF |
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类似说明 - LT3508HFE-PBF |
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