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LM22671QMR-ADJ Folha de dados(PDF) 11 Page - National Semiconductor (TI) |
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LM22671QMR-ADJ Folha de dados(HTML) 11 Page - National Semiconductor (TI) |
11 / 18 page Application Information EXTERNAL COMPONENTS The following design procedures can be used to design a non- synchronous buck converter with the LM22671. Inductor The inductor value is determined based on the load current, ripple current, and the minimum and maximum input voltage. To keep the application in continuous current conduction mode (CCM), the maximum ripple current, I RIPPLE , should be less than twice the minimum load current. The general rule of keeping the inductor current peak-to-peak ripple around 30% of the nominal output current is a good compromise between excessive output voltage ripple and ex- cessive component size and cost. When selecting the induc- tor ripple current ensure that the peak current is below the minimum current limit as given in the Electrical Characteris- tics section. Using this value of ripple current, the value of inductor, L, is calculated using the following formula: where F is the switching frequency which is 500 kHz without an external frequency set resistor or external sync signal ap- plied to the RT/SYNC pin. If the switching frequency is set higher than 500 kHz, the inductance value may not be re- duced accordingly due to stability requirements. The internal compensation is optimized for circuits with a 500 kHz switch- ing frequency. See the internal compensation section for more details. This procedure provides a guide to select the value of the inductor L. The nearest standard value will then be used in the circuit. Increasing the inductance will generally slow down the tran- sient response but reduce the output voltage ripple amplitude. Reducing the inductance will generally improve the transient response but increase the output voltage ripple. The inductor must be rated for the peak current, I PK+, to pre- vent saturation. During normal loading conditions, the peak current occurs at maximum load current plus maximum ripple. Under an overload condition as well as during load transients, the peak current is limited to 700 mA typical (900 mA maxi- mum). This requires that the inductor be selected such that it can run at the maximum current limit and not only the steady state current. Depending on inductor manufacturer, the saturation rating is defined as the current necessary for the inductance to reduce by 30% at 20°C. In typical designs the inductor will run at higher temperatures. If the inductor is not rated for enough current, it might saturate and due to the propagation delay of the current limit circuitry, the power supply may get damaged. Input Capacitor Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch cur- rent during on-time. When the switch turns on, the current into the VIN pin steps to the peak value, then drops to zero at turn- off. The average current into VIN during switch on-time is the load current. The input capacitance should be selected for RMS current, I RMS, and minimum ripple voltage. A good ap- proximation for the required ripple current rating necessary is I RMS > IOUT / 2. Quality ceramic capacitors with a low ESR should be selected for the input filter. To allow for capacitor tolerances and volt- age effects, multiple capacitors may be used in parallel. If step input voltage transients are expected near the maximum rat- ing of the LM22671, a careful evaluation of ringing and pos- sible voltage spikes at the VIN pin should be completed. An additional damping network or input voltage clamp may be required in these cases. Usually putting a higher ESR electrolytic input capacitor in parallel to the low ESR bypass capacitor will help to reduce excessive voltages during a line transient and will also move the resonance frequency of the input filter away from the reg- ulator bandwidth. Output Capacitor The output capacitor can limit the output ripple voltage and provide a source of charge for transient loading conditions. Multiple capacitors can be placed in parallel. Very low ESR capacitors such as ceramic capacitors reduce the output rip- ple voltage and noise spikes, while higher value capacitors in parallel provide large bulk capacitance for transient loading and unloading. Therefore, a combination of parallel capaci- tors, a single low ESR SP or Poscap capacitor, or a high value of ceramic capacitor provides the best overall performance. Output capacitor selection depends on application conditions as well as ripple and transient requirements. Typically a value of at least 100 µF is recommended. An approximation for the output voltage ripple is: In applications with Vout less than 3.3V, where input voltage may fall below the operating minimum of 4.5V, it is critical that low ESR output capacitors are selected. This will limit poten- tial output voltage overshoots as the input voltage falls below device normal operation range. If the switching frequency is set higher than 500 kHz, the ca- pacitance value may not be reduced accordingly due to sta- bility requirements. The internal compensation is optimized for circuits with a 500 kHz switching frequency. See the in- ternal compensation section for more details. Cboot Capacitor The bootstrap capacitor between the BOOT pin and the SW pin supplies the gate current to turn on the N-channel MOS- FET. The recommended value of this capacitor is 10 nF and should be a good quality, low ESR ceramic capacitor. It is possible to put a small resistor in series with the Cboot capacitor to slow down the turn-on transition time of the in- ternal N-channel MOSFET. Resistors in the range of 10 Ω to 50 Ω can slow down the transition time. This can reduce EMI of a switched mode power supply circuit. Using such a series resistor is not recommended for every design since it will in- crease the switching losses of the application and makes thermal considerations more challenging. Resistor Divider For the -5.0 option no resistor divider is required for 5V output voltage. The output voltage should be directly connected to the FB pin. Output voltages above 5V can use the -5.0 option with a resistor divider as an alternative to the -ADJ option. This may offer improved loop bandwidth in some applications. See the Internal Compensation section for more details. For the -ADJ option no resistor divider is required for 1.285V output voltage. The output voltage should be directly con- 11 www.national.com |
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