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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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LM3475MFX Datenblatt(PDF) 9 Page - National Semiconductor (TI) |
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LM3475MFX Datenblatt(HTML) 9 Page - National Semiconductor (TI) |
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9 / 14 page  Design Information SETTING OUTPUT VOLTAGE The output voltage is programmed using a resistor divider between V OUT and GND as shown in Figure 2. The feedback resistors can be calculated as follows: Where Vfb is 0.8V typically. The feedback resistor ratio, α = (R1+R2) / R2, will also be used below to calculate output ripple and operating fre- quency. SETTING OPERATING FREQUENCY AND OUTPUT RIPPLE Although hysteretic control is a simple control scheme, the operating frequency and other performance characteristics depend on external conditions and components. If the induc- tance, output capacitance, ESR, V IN,orCff is changed, there will be a change in the operating frequency and possibly output ripple. Therefore, care must be taken to select com- ponents which will provide the desired operating range. The best approach is to determine what operating frequency is desirable in the application and then begin with the selection of the inductor and output capacitor ESR. The design pro- cess usually involves a few iterations to select appropriate standard values that will result in the desired frequency and ripple. Without the feedforward capacitor (C ff), the operating fre- quency (F) can be approximately calculated using the for- mula: Where delay is the sum of the LM3475 propagation delay time and the PFET delay time. The propagation delay is 90ns typically. Minimum output ripple voltage can be determined using the following equation: V OUT_PP =VHYST (R1+R2)/R2 USING A FEED-FORWARD CAPACITOR The operating frequency and output ripple voltage can also be significantly influenced using a speed up capacitor, C ff,as shown in Figure 2.C ff is connected in parallel with the high side feedback resistor, R1. The output ripple causes a cur- rent to be sourced or sunk through this capacitor. This cur- rent is essentially a square wave. Since the input to the feedback pin (FB) is a high impedance node, the bulk of the current flows through R2. This superimposes a square wave ripple voltage on the FB node. The end result is a reduction in output ripple and an increase in operating frequency. When adding C ff, calculate the formula above with α= 1. The value of C ff depends on the desired operating frequency and the value of R2. A good starting point is 1nF ceramic at 100kHz decreasing linearly with increased operating fre- quency. Also note that as the output voltage is programmed below 1.6V, the effect of C ff will decrease significantly. INDUCTOR SELECTION The most important parameters for the inductor are the inductance and the current rating. The LM3475 operates over a wide frequency range and can use a wide range of inductance values. Minimum inductance can be calculated using the following equation: Where D is the duty cycle, defined as V OUT/VIN, and ∆Iisthe allowable inductor ripple current. Maximum allowable inductor ripple current should be calcu- lated as a function of output current (I OUT) as shown below: ∆I max =IOUT x 0.3 The inductor must also be rated to handle the peak current (I PK) and RMS current given by: I PK =(IOUT + ∆I/2) x 1.1 The inductance value and the resulting ripple is one of the key parameters controlling operating frequency. OUTPUT CAPACITOR SELECTION Once the desired operating frequency and inductance value are selected, ESR must be selected based on the equation in the Setting Operating Frequency and Output Ripple. This process may involve a few iterations to select standard ESR and inductance values. In general, the ESR of the output capacitor and the inductor ripple current create the output ripple of the regulator. How- ever, the comparator hysteresis sets the first order value of this ripple. Therefore, as ESR and ripple current vary, oper- ating frequency must also vary to keep the output ripple 20070115 FIGURE 2. Hysteretic Window www.national.com 9 |
Ähnliche Teilenummer - LM3475MFX |
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Ähnliche Beschreibung - LM3475MFX |
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