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IRU3033CS Datasheet(PDF) 10 Page - International Rectifier |
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IRU3033CS Datasheet(HTML) 10 Page - International Rectifier |
10 / 12 page IRU3033 10 Rev. 1.7 07/17/02 www.irf.com Frequency Calculation The IRU3033 frequency of operation is calculated using the following formula: Where: Vo = Output voltage (V) D = Duty cycle ESR = Output capacitor ESR (V) L = Output inductance ( mH) DVo = Output ripple voltage (V) For our example: Where: Vf = Forward voltage drop of the Schottky diode. The ESR=18m V for 2 of the Sanyo 1500mF, 6MV1500GX caps. If L=3.5 mH then, Fs is calculated as follows: D = (3.38 + 0.5) / 5 = 0.78 Fs = [(Vo3(1 - D)3ESR)] / (L3DVo) (MHz) For example, if Rt=1K and RB=422K, then the output ripple is: The advantage of fixed output ripple is that when the output voltage changes from 2V to 3.5V, the ripple volt- age remains the same which is important in meeting the Intel maximum tolerance specification. Switcher Output Voltage Setting The output voltage of the switcher can be set using the following equations: Assuming, Vo=3.38V and the selected output ripple is ≈ 1.3%(44mV) of the output voltage, a set of equations are derived that selects the resistor divider and the hys- teresis resistor. Assuming, Rt=1K V , 1%: Where: Rt = Top resistor of the resistor divider. RH = Hysteresis resistor connected between pins 3 and 4 of the IRU3033. DVo = Selected output ripple (typically 1% to 2% of output voltage). Assuming, DVo=44mV: Select RH = 249K V, 1% The bottom resistor of the divider is then calculated us- ing the following equations: Where: RB = Bottom resistor of the divider VREF = 1.25V typical Select RB = 576 V, 1% IR WORLD HEADQUARTERS: 233 Kansas St., El Segundo, California 90245, USA Tel: (310) 252-7105 TAC Fax: (310) 252-7903 Visit us at www.irf.com for sales contact information Data and specifications subject to change without notice. 02/01 DVo = (1/422)311 = 26mV RH = (11 3Rt) / DV RH = (11 31000) / 0.044 = 250KV RB = Rt / X X = [(Vo + )/VREF ] - 1 X = [(3.38 + )/1.25 ] - 1 = 1.72 RB = 1000 / 1.72 = 580 V DVo 2 0.044 2 D ≈ (Vo + Vf) / VIN Fs = = 0.087MHz Fs = 87KHz [(3.38 3 (1 - 0.78) 3 0.018)] (3.5 3 0.044) |
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