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LM1770SMF Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM1770SMF Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 14 page  Design Guide (Continued) 20166219 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 ofaC FF capacitor is recommended as it improves the regu- lation and stability of the design. However, its benefit is diminished as V OUT starts approaching VREF , therefore it is not needed in this situation. INPUT CAPACITOR The dominating factor that usually sets an input capacitors’ size is the current handling ability. This is usually determined by the package size and ESR of the capacitor. If these two criteria are met then there usually should be enough capaci- tance to prevent impedance interactions with the source. In general it is recommended to use a ceramic capacitor for the input as they provide a low impedance and small footprint. One important note is to use a good dielectric for the ceramic capacitor such as X5R or X7R. These provide better over temperature performance and also minimize the DC voltage derating that occurs on Y5V capacitors. To calculate the input capacitor RMS current, the equation below can be used: which can be approximated by, MOSFET Selection The two FETs used in the LM1770 requires attention to selection of parameters to ensure optimal performance of the power supply. The high side FET should be a PFET and the low side an NFET. These can be integrated in one package or as two separate packages. The criteria that matter in selection are listed below: VDS VOLTAGE RATING The first selection criteria is to select FETs that have suffi- cient V DS voltage ratings to handle the maximum voltage seen at the input plus any transient spikes that can occur from parasitic ringing. In general most FETs available for this application will have ratings from 8V to 20V. If a larger voltage rating is used then the performance will most likely be degraded because of higher gate capacitance. RDSON The R DS(ON) specification is important as it determines sev- eral attributes of the FET and the overall power supply. The first is that it sets the maximum current of the FET for a given package. A lower R DS(ON) will permit a higher allowable current and reduce conduction losses, however, it will in- crease the gate capacitance and the switching losses. GATE DRIVE The next step is to ensure that the FETs are capable of switching at the low Vin supplies used by the LM1770. The FET should have the Rdson specified at either 1.8V or 2.5V to ensure that it can switch effectively as soon as the LM1770 starts up. GATE CHARGE Because the LM1770 utilizes a fixed dead-time scheme to prevent cross conduction, the FET transitions must occur in this time. The rise and fall time of the FETs gate can be influenced by several factors including the gate capacitance. www.national.com 11 |
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