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LM2698MM-ADJ Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM2698MM-ADJ Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 17 page  Operation (Continued) be connected very close to pin 6 to effectively filter high frequency noise. When operating at 1.25 MHz switching frequency, a minimum bypass capacitance of 0.22 µF is recommended. Output Capacitor The output capacitor in a boost converter provides all the output current when the switch is closed and the inductor is charging. As a result, it sees very large ripple currents. The output capacitor should be capable of handling the maxi- mum RMS current. The RMS current in the output capacitor is: where, and D=(V OUT -VIN)/VOUT The ESR and ESL of the output capacitor directly control the output ripple. Use capacitors with low ESR and ESL at the output for high efficiency and low ripple voltage. Surface mount tantalums, surface mount polymer electrolytic, and polymer tantalum, Sanyo OS-CON, or multi-layer ceramic capacitors are recommended at the output. Compensation This section presents a step-by-step procedure to design the compensation network at pin 1 (V c) of the LM2698. These design methods will produce a conservative and stable con- trol loop. There is a minimum inductance requirement in any current mode converter. This is a function of V OUT, duty cycle, and switching frequency, among other things. The graphs below plot the recommended inductance range vs. duty cycle for V OUT = 12V. The two lines represent the upper and lower bounds of the recommended inductance range. The simpli- fied compensation procedure that follows assumes that the inductance never drops below the Q = 5 line. Figure 4 plots the equation: (1) where, R DSON = 0.15, Se = 0.072*f S, and Q = 0.5 and 5 Use Q = 5 to calculate the minimum inductance recom- mended for a stable design. Choosing an inductor between the Q = 0.5 and Q = 5 values provides a good tradeoff between size and stability. Note that as V IN drops less than 5V, R DS(ON) increases, as shown in the Typical Performance Characteristics section (R DS(ON) vs.VIN curve). The worst case R DS(ON) should be used when choosing the induc- tance. To view plots for different Vout, multiply the Y axis by a factor of V OUT/12, or plot Equation (1) for the respective output voltage. 20012654 20012653 FIGURE 4. Minimum Inductance Requirements for (a) f S = 600kHz and (b) fS = 1.25MHz www.national.com 10 |
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