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LMZ10505TZE-ADJ bảng dữ liệu(PDF) 10 Page - Texas Instruments |
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10 / 28 page Istep x VFB x L x VIN 4 x VOUT x (VIN -VOUT) x 'Vo_tran Co t 'iL = (5V ± 2.5V) x 1.5 PH x 1 MHz 2.5V 5V = 833 mA 'iL = (VIN - VOUT) x D L x fSW 'COUT = 'iL 8 x fSW x ['VOUT ± ('iL x RESR)] ICin(RMS) = IOUT x D(1-D) Cin 8 5A x ( ) x (1 - ) 1 MHz x 50 mV 2.5V 5V 2.5V 5V 8 25 µF D = VOUT VIN LMZ10505 SNVS633G – JANUARY 2010 – REVISED APRIL 2013 www.ti.com where the PWM duty cycle, D, is given by: (2) If ΔVIN is 1% of VIN, this equals to 50 mV and fSW = 1 MHz (3) A second criteria before finalizing the Cin bypass capacitor is the RMS current capability. The necessary RMS current rating of the input capacitor to a buck regulator can be estimated by (4) (5) With this high AC current present in the input capacitor, the RMS current rating becomes an important parameter. The maximum input capacitor ripple voltage and RMS current occur at 50% duty cycle. Select an input capacitor rated for at least the maximum calculated ICin(RMS). Additional bulk capacitance with higher ESR may be required to damp any resonance effects of the input capacitance and parasitic inductance. Output Capacitor Selection In general, 22 µF to 100 µF high quality dielectric (X5R, X7R) ceramic capacitor rated at twice the maximum output voltage is sufficient given the optimal high frequency characteristics and low ESR of ceramic dielectrics. Although, the output capacitor can also be of electrolytic chemistry for increased capacitance density. Two output capacitance equations are required to determine the minimum output capacitance. One equation determines the output capacitance (CO) based on PWM ripple voltage. The second equation determines CO based on the load transient characteristics. Select the largest capacitance value of the two. The minimum capacitance, given the maximum output voltage ripple ( ΔVOUT) requirement, is determined by the following equation: (6) Where the peak to peak inductor current ripple ( ΔiL) is equal to: (7) RESR is the total output capacitor ESR, L is the inductance value of the internal power inductor, where L = 1.5 µH, and fSW = 1 MHz. Therefore, per the design example: (8) The minimum output capacitance requirement due to the PWM ripple voltage is: (9) (10) Three miliohms is a typical RESR value for ceramic capacitors. The following equation provides a good first pass capacitance requirement for a load transient: (11) Where Istep is the peak to peak load step (10% to 90% of the maximum load for this example), VFB = 0.8V, and ΔVo_tran is the maximum output voltage deviation, which is ±20 mV. Therefore the capacitance requirement for the given design parameters is: 10 Submit Documentation Feedback Copyright © 2010–2013, Texas Instruments Incorporated Product Folder Links: LMZ10505 |
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