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SC4508ABUCKEVB Datasheet(PDF) 11 Page - Semtech Corporation |
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SC4508ABUCKEVB Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 21 page  11 2007 Semtech Corp. www.semtech.com POWER MANAGEMENT SC4508A 0 5 10 15 20 0 20 40 FOM:100*10^{-12} FOM:200*10^{-12} FOM:500*10^{-12} On-resistance (mOhm) 50 1 Cg 100 Rds , () Cg 200 Rds , () Cg 500 Rds , () 20 1 Rds Figure 5. Figure of Merit curves. The closer the curve is to the origin, the lower is the FOM. This means lower switching loss or lower conduction loss or both. It may be difficult to find MOSFETs with both low C g and low Rds(on. Usually a trade-off between Rds(on and C g has to be made. MOSFET selection also depends on applications. In many applications, either switching loss or conduction loss dominates for a particular MOSFET. For buck and buck- boost converters with high input to output voltage ratios, the MOSFET is hard switched but conducts with very low duty cycle. For such applications, MOSFETs with low C g should be used. MOSFET power dissipation consists of: a) conduction loss due to the channel resistance R ds(on), b) switching loss due to the switch rise time t r and fall time t f, and c) the gate loss due to the gate resistance R G. The RMS value of the MOSFET switch current is calculated as: ) 1 ( D I I 12 dc Qrms 2 δ + = The conduction losses are then P tc = IQrms 2 R ds(on) Idc is average inductor current. In buck converter, it is also load current. In buck-boost, it is load current divided by 1-D. R ds(on) varies with temperature and gate-source voltage. Curves showing R ds(on) variations can be found in manufacturers’ data sheet. From the FDS6675 datasheet, R ds(on) is less than 14mΩ when Vgs is greater than 10V. However R ds(on) increases by 30% as the junction temperature increases from 25oC to 110oC. The switching losses can be estimated using the simple formula: s in dc 2 f r 2 1 ts f V I) 1 )( t t ( P δ + + = where t r is the rise time and tf is the fall time of the switching process. Different manufacturers have different definitions and test conditions for t r and t f . To clarify these, we sketch the typical MOSFET switching characteristics under clamped inductive mode in Figure 6. Figure 6. MOSFET switching characteristics In Figure 6, Q gs1 is the gate charge needed to bring the gate-to-source voltage V gs to the threshold voltage Vgs_th, Q gs2 is the additional gate charge required for the switch current to reach its full-scale value I ds . and Q gd is the charge needed to charge gate-to-drain (Miller) capacitance when V ds is falling. Switching losses occur during the time interval [t 1, t3]. Defining t r = t3-t1 and tr can be approximated as: gsp cc gt gd 2 gs r V V R ) Q Q ( t − + = where R gt is the total resistance from the driver supply rail to the gate of the MOSFET. It includes the gate driver internal impedance R gi, external resistance Rge and the gate resistance R g within the MOSFET i.e. R gt = Rgi+Rge+Rg V gsp is the Miller plateau voltage shown in Figure 11. Similarly an approximate expression for t f is: Application Information (Cont.) |
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