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LM3224MMX-ADJ Scheda tecnica(PDF) 10 Page - National Semiconductor (TI) |
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LM3224MMX-ADJ Scheda tecnica(HTML) 10 Page - National Semiconductor (TI) |
10 / 18 page Operation (Continued) INTRODUCTION TO COMPENSATION The LM3224 is a current mode PWM boost converter. The signal flow of this control scheme has two feedback loops, one that senses switch current and one that senses output voltage. To keep a current programmed control converter stable above duty cycles of 50%, the inductor must meet certain criteria. The inductor, along with input and output voltage, will determine the slope of the current through the inductor (see Figure 2 (a)). If the slope of the inductor current is too great, the circuit will be unstable above duty cycles of 50%. A 10µH to 15µH inductor is recommended for most 615 kHz applications, while a 4.7µH to 10µH inductor may be used for most 1.25 MHz applications. If the duty cycle is approaching the maximum of 85%, it may be necessary to increase the inductance by as much as 2X. See Inductor and Diode Selection for more detailed inductor sizing. The LM3224 provides a compensation pin (V C) to customize the voltage loop feedback. It is recommended that a series combination of R C and CC be used for the compensation network, as shown in the typical application circuit. For any given application, there exists a unique combination of R C and C C that will optimize the performance of the LM3224 circuit in terms of its transient response. The series combi- nation of R C and CC introduces a pole-zero pair according to the following equations: where R O is the output impedance of the error amplifier, approximately 900k Ω. For most applications, performance can be optimized by choosing values within the range 5k Ω≤ R C ≤ 100kΩ (R C can be up to 200k Ω if C C2 is used, see High Output Capacitor ESR Compensation) and 680pF ≤ C C ≤ 10nF. Refer to the Applications Information section for rec- ommended 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 differ- ent 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 continu- ous conduction operation, 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 pro- duce 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 equa- tion is: where fs is the switching frequency, D is the duty cycle, and R DSON is the ON resistance of the internal switch taken from the graph "NMOS R DSON vs. Input Voltage" 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 2 (a) is given by: 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 I LOAD/D’) plus ∆i L. As a side note, discontinuous operation occurs when the inductor current falls to zero during a switching cycle, or ∆i L is greater than the average inductor current. Therefore, con- tinuous conduction mode occurs when ∆i L 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 con- tinuous conduction mode is shown in Figure 2 (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 condi- tions are possible in the application, the diode current rating 20097605 FIGURE 2. (a) Inductor current. (b) Diode current. www.national.com 10 |
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