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LM1771TSDX Folha de dados(PDF) 11 Page - National Semiconductor (TI) |
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LM1771TSDX Folha de dados(HTML) 11 Page - National Semiconductor (TI) |
11 / 17 page Design Guide (Continued) 20189029 This circuit uses an additional resistor in series with the inductor to add ripple at the output. It is placed in this location and used in combination with the feed-forward capacitor (C FF) to provide ripple to the feedback pin, without adding ripple or a DC offset to the output. The benefit of using a ceramic capacitor is still obtained with this technique. Be- cause the addition of the resistor results in power loss, this circuit implementation is only recommended for low currents (2A and below). The power loss and rating of the resistor should be taken into account when selecting this compo- nent. This circuit implementation utilizing the feed-forward capaci- tor begins to experience limitations when the output voltage is small. Previously the circuit relied on the C FF for all the ripple at the feedback node by assuming that the resistor divider was negligible. As V OUT decreases this can not be assumed. The resistor divider contributes a larger amount of ripple which is problematic as it is also out of phase. There- fore the resistor location should be changed to be in series with the output capacitor. This can be viewed as adding an effective ESR to the output capacitor. 20189030 FEED-FORWARD CAPACITOR The feed-forward capacitor is used across the top feedback resistor to provide a lower impedance path for the high frequency ripple without degrading the DC accuracy. Typi- cally the value for this capacitor should be small enough to prevent load transient errors because of the discharging time, but large enough to prevent attenuation of the ripple voltage. In general a small ceramic capacitor in the range of 1nF to 10nF is sufficient. If C FF is used then it can be assumed that the ripple voltage seen at the feedback pin is the same as the ripple voltage at the output. The attenuation factor H no longer needs to be used. However, in these conditions, it is recommended to have a minimum of 20mV ripple at the feedback pin. The use www.national.com 11 |
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