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LTC3854EMSE 数据表(PDF) 21 Page - Linear Technology |
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LTC3854EMSE 数据表(HTML) 21 Page - Linear Technology |
21 / 28 page LTC3854 3854fa applicaTions inForMaTion However, the amount of capacitance needed is determined not only by the allowed ripple in steady state but by the maximum energy stored in the inductor. The capacitance must be sufficient in value to absorb the change in induc- tor current when a high current to low current transient occurs. The minimum capacitance to assure the inductor’s energy is adequately absorbed during a 5A load step for a maximum overshoot of 2% is: COUT ≥ L • ∆IL2 2 • ∆VOUT • VOUT COUT ≥ 0.56µH •(5A)2 0.02 • 1.2V COUT ≥583µF A maximum overshoot or undershoot of 2% for a 5A load step will require an ESR of: ESR < 0.02 • VOUT ∆ILOAD = 0.02 • 1.2V 5A ≤ 5mΩ Several quality capacitors are available with low enough ESR. Multilayer ceramic capacitors tend to have very low ESR values.ItisalsoagoodpracticetoreducetheESLbyputting several capacitors in parallel on the output (a parallel bank of larger and smaller capacitors will improve performance in both a DC and a transient condition). To keep ripple very low and design for any possible large excursions in current 2x 330µF (tantalum or polymer surface) and 1x 47µF polymer low ESR type were con- nected in parallel. Choosing FB Resistors (See Figure 3) VOUT = 0.8 1+ RB RA RB = 0.5RA Using 1% 10.0k for RA gives 1% 4.99k for RB. Choosing CIN Capacitors CINischosenforaRMScurrentratingofatleastIOUT(MAX)/2 = 6A. Again, keeping ESR low will improve performance and reduce power loss (several capacitors in parallel is once again a good choice). We will use an 180µF 25V electrolytic with 2x 10µF 25V low ESR ceramic capacitors connected in parallel. Choosing MOSFETs The power dissipation in the main and synchronous FETs canbeeasilyestimated.ChoosingaRenesasRJK0305DPB for the main FET results in the following parameters: BVDSS = 30V RDS(ON) = 13mΩ maximum at 25°C, VGS = 4.5V QGD=1.5nCatVDS,test10VresultsinCMILLER=1.5nC/10V = 150pF QG = 8nC, typical, at VGS = 4.5V VMILLER = 2.8V At VIN = 20V, IOUT = 15A, estimated TJ = 100°C for the top FET and given VINTVCC = 5.0V RDR,PULLUP = 2.6Ω RDR,PULLDOWN = 1.5Ω the total losses in the main FET will be: PMAIN = 1.2V 20V •(15A)2 • 1 + 0.005 • 100°C – 25°C ( ) ( ) • 13m Ω + 20V ( )2 • 15A 2 • 150pF • 2.5 Ω 5V − 2.8V + 1.2 Ω 2.8V • fSW PMAIN = 0.55W |
类似零件编号 - LTC3854EMSE |
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类似说明 - LTC3854EMSE |
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