Datenblatt-Suchmaschine für elektronische Bauteile |
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LM3475 Datenblatt(PDF) 10 Page - Texas Instruments |
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LM3475 Datenblatt(HTML) 10 Page - Texas Instruments |
10 / 21 page 'I 2 3 IOUT 2 + IRMS = L = VIN - VSD - VOUT 'I x F D F = (VIN - VOUT) x ESR x (VHYST x D x L) + (VIN x delay x ESR) VIN VOUT LM3475 SNVS239A – OCTOBER 2004 – REVISED JANUARY 2005 www.ti.com 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 inductance, output capacitance, ESR, VIN, or Cff is changed, there will be a change in the operating frequency and possibly output ripple. Therefore, care must be taken to select components 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 process usually involves a few iterations to select appropriate standard values that will result in the desired frequency and ripple. Without the feedforward capacitor (Cff), the operating frequency (F) can be approximately calculated using the formula: where • Delay is the sum of the LM3475 propagation delay time and the PFET delay time • The propagation delay is 90ns typically (4) Minimum output ripple voltage can be determined using the following equation: VOUT_PP = VHYST ( R1 + R2 ) / R2 (5) USING A FEED-FORWARD CAPACITOR The operating frequency and output ripple voltage can also be significantly influenced using a speed up capacitor, Cff, as shown in Figure 13. Cff is connected in parallel with the high side feedback resistor, R1. The output ripple causes a current to be sourced or sunk through this capacitor. This current 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 Cff, calculate the formula above with α= 1. The value of Cff 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 frequency. Also note that as the output voltage is programmed below 1.6V, the effect of Cff 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 VOUT/VIN • ΔI is the allowable inductor ripple current (6) Maximum allowable inductor ripple current should be calculated as a function of output current (IOUT) as shown below: ΔImax = IOUT x 0.3 The inductor must also be rated to handle the peak current (IPK) and RMS current given by: IPK = (IOUT + ΔI/2) x 1.1 (7) (8) The inductance value and the resulting ripple is one of the key parameters controlling operating frequency. 10 Submit Documentation Feedback Copyright © 2004–2005, Texas Instruments Incorporated Product Folder Links: LM3475 |
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