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BD95700MUV Datasheet(PDF) 8 Page - Rohm |
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BD95700MUV Datasheet(HTML) 8 Page - Rohm |
8 / 21 page BD95700MUV Technical Note 8/20 www.rohm.com 2009.04 - Rev.B © 2009 ROHM Co., Ltd. All rights reserved. ● External Component Selection 1. Inductor (L) selection ※ Passing a current larger than the inductor’s rated current will cause magnetic saturation in the inductor and decrease system efficiency. In selecting the inductor, be sure to allow enough margin to assure that peak current does not exceed the inductor rated current value. ※ To minimize possible inductor damage and maximize efficiency, choose a inductor with a low (DCR, ACR) resistance. 2. Output Capacitor (CO) Selection Please give due consideration to the conditions in formula (7) below for output capacity, bearing in mind that output rise time must be established within the soft start time frame. Note: Improper capacitor may cause startup malfunctions. 3. Input Capacitor (Cin) Selection A low ESR capacitor is recommended to reduce ESR loss and maximize efficiency. Δ IL= (VIN-VOUT) x VOUT L x VIN x f [A]・・・(3) Δ IL=0.3×IOUTmax/2. [A]・・・(4) L= (VIN-VOUT) x VOUT Δ IL x VIN x f [H]・・・(5) At least 20mV ripple voltage of the FB voltage is recommended by taking the equivalent series resistance and inductance into account. Output ripple voltage is determined as in formula (6) below. Δ VOUT=ΔIL×ESR+ESL×ΔIL/TON・・・(6) Co≦ 3msec×(Limit-IOUT/2) VOUT ・・・ (7) Input Capacitor IRMS= VOUT(VIN-VOUT) VIN [A]・・・(8) √ IOUT 4 Δ IL VIN IL L Co VOUT Output Ripple Current VIN L Co VOUT Cin VIN L Co VOUT ESR Output Capacitor ESL The inductor value is a major influence on the output ripple current. As formula (3) below indicates, the greater the inductor or the switching frequency, the lower the ripple current. The proper output ripple current setting is about 30% of maximum output current. (ΔIL: output ripple current; f: switch frequency) (ΔIL: Output ripple current; ESR: CO equivalent series resistance, ESL:equivalent series inductance) ※ In selecting a capacitor, make sure the capacitor rating allows sufficient margin relative to output voltage. Note that a lower ESR can minimize output ripple voltage. Limit: Current Limit Value The input capacitor selected must have low enough ESR resistance to fully support large ripple output, in order to prevent extreme over current. The formula for ripple current IRMS is given in (8) below. Where VIN=2×VOUT,IRMS= x 2 IOUT |
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