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LM2700 датащи(PDF) 11 Page - Texas Instruments |
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LM2700 датащи(HTML) 11 Page - Texas Instruments |
11 / 26 page 'iL = (in Amps) VIND 2Lfs VINRDSON 0.144 fs L > ( ) D D' 2 -1 ( ) D D' +1 (in H) LM2700 www.ti.com SNVS152C – MAY 2001 – REVISED MARCH 2013 where RO is the output impedance of the error amplifier, approximately 850kΩ. For most applications, performance can be optimized by choosing values within the range 5k Ω ≤ RC ≤ 20kΩ (RC can be up to 200kΩ if CC2 is used, see High Output Capacitor ESR Compensation) and 680pF ≤ CC ≤ 4.7nF. Refer to the Application Information section for recommended values for specific circuits and conditions. Refer to the COMPENSATION section for other design requirement. COMPENSATION This section will present a general design procedure to help insure a stable and operational circuit. The designs in this datasheet are optimized for particular requirements. If different conversions are required, some of the components may need to be changed to ensure stability. Below is a set of general guidelines in designing a stable circuit for continuous conduction operation (loads greater than approximately 100mA), in most all cases this will provide for stability during discontinuous operation as well. The power components and their effects will be determined first, then the compensation components will be chosen to produce stability. INDUCTOR AND DIODE SELECTION Although the inductor sizes mentioned earlier are fine for most applications, a more exact value can be calculated. To ensure stability at duty cycles above 50%, the inductor must have some minimum value determined by the minimum input voltage and the maximum output voltage. This equation is: (5) where fs is the switching frequency, D is the duty cycle, and RDSON is the ON resistance of the internal switch taken from the graph "RDSON vs. VIN" in the Typical Performance Characteristics section. This equation is only good for duty cycles greater than 50% (D>0.5), for duty cycles less than 50% the recommended values may be used. The corresponding inductor current ripple as shown in Figure 18(a) is given by: (6) The inductor ripple current is important for a few reasons. One reason is because the peak switch current will be the average inductor current (input current or ILOAD/D') plus ΔiL. As a side note, discontinuous operation occurs when the inductor current falls to zero during a switching cycle, or ΔiL is greater than the average inductor current. Therefore, continuous conduction mode occurs when ΔiL is less than the average inductor current. Care must be taken to make sure that the switch will not reach its current limit during normal operation. The inductor must also be sized accordingly. It should have a saturation current rating higher than the peak inductor current expected. The output voltage ripple is also affected by the total ripple current. The output diode for a boost regulator must be chosen correctly depending on the output voltage and the output current. The typical current waveform for the diode in continuous conduction mode is shown in Figure 18(b). The diode must be rated for a reverse voltage equal to or greater than the output voltage used. The average current rating must be greater than the maximum load current expected, and the peak current rating must be greater than the peak inductor current. During short circuit testing, or if short circuit conditions are possible in the application, the diode current rating must exceed the switch current limit. Using Schottky diodes with lower forward voltage drop will decrease power dissipation and increase efficiency. DC GAIN AND OPEN-LOOP GAIN Since the control stage of the converter forms a complete feedback loop with the power components, it forms a closed-loop system that must be stabilized to avoid positive feedback and instability. A value for open-loop DC gain will be required, from which you can calculate, or place, poles and zeros to determine the crossover frequency and the phase margin. A high phase margin (greater than 45°) is desired for the best stability and transient response. For the purpose of stabilizing the LM2700, choosing a crossover point well below where the right half plane zero is located will ensure sufficient phase margin. A discussion of the right half plane zero and checking the crossover using the DC gain will follow. Copyright © 2001–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2700 |
Аналогичный номер детали - LM2700_15 |
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Аналогичное описание - LM2700_15 |
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