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BD8966FVM Datasheet(PDF) 10 Page - Rohm |
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BD8966FVM Datasheet(HTML) 10 Page - Rohm |
10 / 15 page BD8966FVM Technical Note 10/14 www.rohm.com 2009.05 - Rev.A © 2009 ROHM Co., Ltd. All rights reserved. 2. Selection of output capacitor (CO) As the output rise time must be designed to fall within the soft-start time, the capacitance of output capacitor should be determined with consideration on the requirements of equation (5): In case of BD8966FVM, for instance, and if VOUT=1.5V, IOUT=0.8A, and TSS=1ms, Inappropriate capacitance may cause problem in startup. A 10 μF to 100 μF ceramic capacitor is recommended. 3. Selection of input capacitor (Cin) A low ESR 10μF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better efficiency. Input capacitor to select must be a low ESR capacitor of the capacitance sufficient to cope with high ripple current to prevent high transient voltage. The ripple current IRMS is given by the equation (5): IRMS=IOUT× VOUT(VCC-VOUT) VCC [A]・・・(5) √ When Vcc is twice the VOUT, IRMS= IOUT 2 < Worst case > IRMS(max.) Fig.27 Input capacitor VCC L Co VOUT Cin If VCC=5.0V, VOUT=1.5V, and IOUTmax.=0.8A Output capacitor should be selected with the consideration on the stability region and the equivalent series resistance required to smooth ripple voltage. Output ripple voltage is determined by the equation (4): ΔVOUT=ΔIL×ESR [V]・・・(4) (ΔIL: Output ripple current, ESR: Equivalent series resistance of output capacitor) *Rating of the capacitor should be determined allowing sufficient margin against output voltage. Less ESR allows reduction in output ripple voltage. Fig.26 Output capacitor Co≦ TSS×(Ilimit-IOUT) VOUT ・・・ (5) Tss: Soft-start time Ilimit: Over current detection level, 2A(Typ) VCC L Co VOUT ESR Co≦ 1m×(2-0.8) 1.5 ≒ 800[μF] IRMS=0.8× 5(5-1.5) 5 =0.67[ARMS] √ |
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