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ISL6506BCBZ Datasheet(PDF) 7 Page - Intersil Corporation |
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ISL6506BCBZ Datasheet(HTML) 7 Page - Intersil Corporation |
7 / 8 page 7 FN9141.6 September 12, 2013 current demands. Thus, it is recommended that the output capacitors be selected for transient load regulation, paying attention to their parasitic components (ESR, ESL). Also, during the transition between active and sleep states on the 5VDUAL output, there is a short interval of time during which none of the power pass elements are conducting. During this time the output capacitors have to supply all the output current. The output voltage drop during this brief period of time can be easily approximated using Equation 1: where: ΔV OUT = output voltage drop ESROUT = output capacitor bank ESR IOUT = output current during transition COUT = output capacitor bank capacitance tt = active-to-sleep/sleep-to-active transition time (10µs typical) The output voltage drop is heavily dependent on the ESR (equivalent series resistance) of the output capacitor bank, the choice of capacitors should be such as to maintain the output voltage above the lowest allowable regulation level. Input Capacitors Selection The input capacitors for an ISL6506, ISL6506A application must have a sufficiently low ESR so as not to allow the input voltage to dip excessively when energy is transferred to the output capacitors. If the ATX supply does not meet the specifications, certain imbalances between the ATX’s outputs and the ISL6506, ISL6506A’s regulation levels could have as a result a brisk transfer of energy from the input capacitors to the supplied outputs. At the transition between active and sleep states, such phenomena could be responsible for the 5VSB voltage drooping excessively and affecting the output regulation. The solution to such a potential problem is using larger input capacitors with a lower total combined ESR. Transistor Selection/Considerations The ISL6506, ISL6506A usually requires one P-Channel and two N-Channel MOSFETs. All three of these MOSFETs are utilized as ON/OFF switching elements. One important criteria for selection of transistors for all the switching elements is package selection for efficient removal of heat. The power dissipated in a switch element while on is shown in Equation 2: Select a package and heatsink that maintains the junction temperature below the rating with the maximum expected ambient temperature. Q1, Q3 These N-Channel MOSFETs are used to switch the 3.3V and 5V inputs provided by the ATX supply into the 3.3VAUX and 5VDUAL outputs while in active (S0, S1) state. The main criteria for the selection of these transistors is output voltage budgeting. The maximum rDS(ON) allowed at highest junction temperature can be expressed using Equation 3: where: VINmin = minimum input voltage VOUTmin = minimum output voltage allowed IOUTmax = maximum output current Q2 This is a P-Channel MOSFET used to switch the 5VSB output of the ATX supply into the 5VDUAL output during sleep states. The selection criteria of this device, as with the N-Channel MOSFETs, is proper voltage budgeting. The maximum rDS(ON), however, has to be achieved with only 4.5V of gate-to-source voltage, so a true logic level MOSFET needs to be selected. V OUT Δ I OUT ESR OUT t t C OUT ---------------- + ⎝⎠ ⎜⎟ ⎛⎞ × = (EQ. 1) P LOSS Io 2 rDS ON () × = (EQ. 2) r DS ON ()max V INmin V OUTmin – I OUTmax --------------------------------------------------- = (EQ. 3) ISL6506, ISL6506A, ISL6506B |
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