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TPS40140 Datasheet(PDF) 42 Page - Texas Instruments |
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TPS40140 Datasheet(HTML) 42 Page - Texas Instruments |
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42 / 71 page  diode OUT d f sw P = 2 × I × t × V × f = 0.77 W P SRcond + ISRrms 2 R DS(on) (sr) N + 0.7 W I SRrms + (1 * D) I OUT 2 ) I RIPPLE 2 12 + 18.7 A P SWtot + PSWcond ) PSWsw + 0.91 W sw drv sw sw sw Ipk × Vin f R Qgd Qgs Psw 0.26 W Vgtdrv × × × ( + ) = = P SWcond + ISWrms 2 R DS(on) (sw) + 0.65 W I SWrms + D I OUT 2 ) I RIPPLE 2 12 + 7.07 A RMS_CIN OUT I = D × (1 - D) × I TPS40140 SLUS660I – SEPTEMBER 2005 – REVISED JANUARY 2015 www.ti.com Another important consideration for the input capacitor is the RMS ripple current rating. The RMS current in the input capacitor is estimated by: (27) 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. 9.2.1.2.4 Step 4: MOSFET Selection 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. The RENESAS RJK0305 and RJK0301 are selected as the high-side and low-side MOSFETs respectively. To reduce the conduction loss, two RJK0301 components are used. The power losses in the high-side MOSFET is calculated with the following equations: The RMS current in the high side MOSFET is show in Equation 28. (28) The RDS(on) is 13 mΩ when the MOSFET gate voltage is 4.5 V. The conduction loss is: (29) The switching loss is: (30) The calculated total loss in the high-side MOSFET is: (31) The power losses in the low-side SR MOSFET is calculated in the following equations: The RMS current in the low-side MOSFET is shown in Equation 32. (32) The RDS(on) is 4 mΩ when the MOSFET gate voltage is 4.5 V. The total conduction loss in the two low-side MOSFETs is shown in Equation 33. where • N is the number of MOSFETs. Here, it is equal to 2. (33) The total power loss in the body diode is: (34) So the calculated total loss in the SR MOSFET is: 42 Submit Documentation Feedback Copyright © 2005–2015, Texas Instruments Incorporated Product Folder Links: TPS40140 |
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