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SC2677BITSTRT Datasheet(PDF) 8 Page - Semtech Corporation |
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SC2677BITSTRT Datasheet(HTML) 8 Page - Semtech Corporation |
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8 / 19 page  8 © 2009 Semtech Corp. www.semtech.com POWER MANAGEMENT SC2677B Over Current Protection Over Current Protection Over Current Protection Over Current Protection Over Current Protection Current sense amplifiers sense the inductor DCR, and com- pare with an internal OCP reference. As over current being detected, the current sense amplifier will trip the peak current limit on cycle-by-cycle basis. If the over current condition sustains, and the output voltage drops below 75% of its nominal voltage level, the PWM will be disabled and the power supply be latched off with short amount of delay. The latch can be reset by power cycling. (R13+R19) vs.Oscillator Frequency 300 400 500 600 700 800 900 1000 4 6 8 1012 1416 1820 (R13+R19) (kohm) Vphasing vs Phase Shift 0 20 40 60 80 100 120 140 160 180 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 Vphasing (V) Controller Power Dissipation Controller Power Dissipation Controller Power Dissipation Controller Power Dissipation Controller Power Dissipation Controller power dissipation is generated by following parameter; switching frequency, total gate charge of all selected MOSFETs and supply voltage. P = Vin * (I CC + QGT* FSW) Q GT = QG * N Where Vin : Supply voltage for controller and driving MOSFET. Layout Guidelines Layout Guidelines Layout Guidelines Layout Guidelines Layout Guidelines Power and signal traces must be kept separated for noise considerations. Feedback, current sense traces and ana- log ground should not cross any traces or planes carrying high switching currents, such as in the input loop or the phase node. The input loop, consisting of the input capacitors and both MOSFETs must be kept as small as possible. Since all of the high switching currents occur in the input loop, the enclosed loop area must be kept small to minimize induc- tance and radiated and conducted noise emissions. An example is shown below to demonstrate the procedure introduced above. Vin =12V Fsw =250KHz N =4(number of MOSFET) Then Q GT = 108nC Q G = 27nC (per MOSFET) It’s recommended that the below figure be performed to ensure SC2677B under safe operation area. I CC : Supply current for controller. Q GT : Total gate charge of all selected MOSFETs. Q G : Total gate charge of per selected MOSFETs. F SW : Switching frequency. N : Number of MOSFET. QGT limitation (with loading) 20 60 100 140 180 220 260 300 340 380 420 460 500 540 580 620 150 200 250 300 350 400 450 500 Fsw(KHz) 5Vin 8.5Vin 12Vin SOA SOA SOA Application Information(Cont.) |
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