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LM2696 Arkusz danych(PDF) 11 Page - Texas Instruments |
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LM2696 Arkusz danych(HTML) 11 Page - Texas Instruments |
11 / 28 page Cff_MAX = (VIN_MIN - VFB) x TON_MIN 0.03V x Rff pF Rff RFB1 RFB2 Cff SW FB L COUT VOUT H = VOUT VFB = VOUT 1.25V LM2696 www.ti.com SNVS375B – OCTOBER 2005 – REVISED APRIL 2013 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: ΔVFB > −0.057 • fSW + 35 where • fSW is in kHz • ΔVFB is in mV. (9) 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 feedback resistors. This gain H may be expressed as: (10) 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 18. Figure 18. RC Feed Forward Network The value of Rff should be large in order to prevent any potential offset in VOUT. Typically the value of Rff is on the order of 1 M Ω and the value of RFB1 should be less than 10 kΩ. The large difference in resistor values minimizes output voltage offset errors in DCM. The value of the capacitor may be selected using the following relationship: where • on-time (TON_MIN) is in µs • resistance (Rff) is in MΩ. (11) Copyright © 2005–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2696 |
Podobny numer części - LM2696_15 |
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Podobny opis - LM2696_15 |
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