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TPS40180 Datasheet(PDF) 36 Page - Texas Instruments |
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TPS40180 Datasheet(HTML) 36 Page - Texas Instruments |
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36 / 52 page  www.ti.com RIPPLE RIPPLE(TotOUT) RIPPLE(TotOUT) RIPPLE(COUT) OUT SW Co RIPPLE RIPPLE I V V V 8 C f ESR I I æ ö - ç ÷ - ´ ´ è ø = = (22) Step 3: Input Capacitor Selection f OUT OUT IN(min) RIPPLE(Cin) IN SW I V C V V ´ = ´ ´ (23) RIPPLE(CinESR) Cin 1 OUT RIPPLE 2 V ESR I I = + (24) RMS _ CIN OUT I D (1 D) I = ´ - ´ (25) MOSFET Selection 2 2 RIPPLE rms OUT I Isw D I 7.08 A 12 æ ö ç ÷ = ´ + = ç ÷ è ø (26) ( ) 2 SW(cond) SWrms DS(on)(sw ) P I R 0.47 W = ´ = (27) ( ) PK IN SW DRV SW SW SW(sw ) I V f R Qgd Qgs P 0.35 W Vgtdrv ´ ´ ´ ´ + = = (28) TPS40180 SLVS753B – FEBRUARY 2007 – REVISED NOVEMBER 2007 With an 880- µF output capacitance, the ripple voltage at the capacitor is calculated to be 1.5 mV. In the specification, the output ripple voltage should be less than 30 mV, so based on Equation 22, the required maximum ESR is 9.4 m Ω. The selected capacitors can meet this requirement. The input voltage ripple depends on input capacitance and ESR. The minimum capacitor and the maximum ESR can be estimated by: For this design, assume VRIPPLE(Cin) is 100 mV and VRIPPLE(CinESR) is 50 mV, so the calculated minimum capacitance is 89 µF and the maximum ESR is 2.3 mΩ. Choosing four 22 µF, 16 V, 2mΩ ESR ceramic capacitors meets this requirement. Another important thing for the input capacitor is the RMS ripple current rating. The RMS current in the input capacitor is estimated by: where • D is the duty cycle The calculated RMS current is 6.6 A. Each selected ceramic capacitor has a RMS current rating of 4.3 A, so it is sufficient to reach this requirement. The MOSFET selection determines the converter efficiency. In this design, the duty cycle is very small so that the high-side MOSFET is dominated with switching losses and the low-side MOSFET is dominated with conduction loss. To optimize the efficiency, choose smaller gate charge for the high-side MOSFET and smaller RDS(on) for the low-side MOSFET. RENESAS HAT2167H and HAT2164H are selected as the high-side and low-side MOSFET respectively. The power losses in the high-side MOSFET is calculated with the following equations: The RMS current in the high-side MOSFET is The RDS(on)(sw) is 9.3 mΩ when the MOSFET gate voltage is 4.5 V. The conduction loss is: The switching loss is: The calculated total loss is the high-side MOSFET is: 36 Submit Documentation Feedback Copyright © 2007, Texas Instruments Incorporated Product Folder Link(s): TPS40180 |
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