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LM2696MXA Scheda tecnica(PDF) 9 Page - National Semiconductor (TI) |
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LM2696MXA Scheda tecnica(HTML) 9 Page - National Semiconductor (TI) |
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9 / 19 page  Application Information (Continued) OUTPUT CAPACITOR The output capacitor size and ESR have a direct affect on the stability of the loop. This is because the adaptive on-time control scheme works by sensing the output voltage ripple and switching appropriately. The output voltage ripple on a buck converter can be approximated by assuming that the AC inductor ripple current flows entirely into the output ca- pacitor and the ESR of the capacitor creates the voltage ripple. This is expressed as: ∆V OUT ≈∆I L • RESR To ensure stability, two constraints need to be met. These constraints are the voltage ripple at the feedback pin must be greater than some minimum value and the voltage ripple must be in phase with the switch pin. The ripple voltage necessary at the feedback pin may be estimated using the following relationship: ∆V FB > −0.057 • fSW +35 Where f SW is in kHz and ∆V FB is in mV. This minimum ripple voltage is necessary in order for the comparator to initiate switching. The voltage ripple at the feedback pin must be in-phase with the switch. Because the ripple due to the capacitor charging and capacitor ESR are out of phase, the ripple due to capacitor ESR must dominate. The ripple at the output may be calculated by multiplying the feedback ripple voltage by the gain seen through the feed- back resistors. This gain H may be expressed as: To simplify design and eliminate the need for high ESR output capacitors, an RC network may be used to feed forward a signal from the switchpin to the feedback (FB) pin. See the ‘Ripple Feed Forward’ section for more details. Typically, the best performance is obtained using POSCAPs, SP CAPs, tantalum, Niobium Oxide, or similar chemistry type capacitors. Low ESR ceramic capacitors may be used in conjunction with the RC feed forward scheme; however, the feed forward voltage at the feedback pin must be greater than 30 mV. RIPPLE FEED FORWARD An RC network may be used to eliminate the need for high ESR capacitors. Such a network is connected as shown in Figure 1. The value of R ff should be large in order to prevent any potential offset in V OUT. Typically the value of Rff is on the order of 1 M Ω and the value of R FB1 should be less than 10 k Ω. The large difference in resistor values minimizes output voltage offset errors in DCM. The value of the capaci- tor may be selected using the following relationship: Where the on-time (T ON_MIN) is in µs, and the resistance (R ff)isinM Ω. FEEDBACK RESISTORS The feedback resistors are used to scale the output voltage to the internal reference value such that the loop can be regulated. The feedback resistors should not be made arbi- trarily large as this creates a high impedance node at the feedback pin that is more susceptible to noise. Typically, R FB2 is on the order of 1 k Ω. To calculate the value of R FB1, one may use the relationship: Where V FB is the internal reference voltage that can be found in the electrical characteristics table (1.254V typical). The output voltage value can be set in a precise manner by taking into account the fact that the reference voltage is regulating the bottom of the output ripple as opposed to the average value. This relationship is shown in Figure 2. 20153428 FIGURE 1. RC Feed Forward Network www.national.com 9 |
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