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LTM4650-1 Datasheet(PDF) 11 Page - Linear Technology |
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LTM4650-1 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 36 page  LTM4650-1 11 46501fc For more information www.linear.com/LTM4650-1 The typical LTM4650-1 application circuit is shown in Figure 24. External component selection is primarily determined by the maximum load current and output voltage. Refer to Table 4 for specific external capacitor requirements for particular applications. OUTPUT TOTAL DC ACCURACY AND AC TRANSIENT PERFORMANCE In modern ASIC and FPGA power supply designs, a tight total voltage regulation window, ±3% for example, is re- quired of the supply powering the core and periphery. To meet this requirement, the supply’s DC voltage variance plus any AC voltage variation which may occur during any load step transient must fall within this allowed window. The DC voltage variance is determined by the accuracies of the supply’s reference voltage, resistor divider, load regulation and line regulation over the operating tempera- ture range. The AC voltage variance is determined by the supply’s output voltage overshoot and undershoots in response to a load transient condition for a given output capacitor network. Figure 2 shows a typical load step transient response waveform together with DC voltage accuracy variance. For a given allowable voltage regulation window, a tighter DC voltage accuracy allows more margin for the AC variation due to a load transient response. This increased margin for AC variation allows for a reduction in the total output capacitance required to meet the regulation window re- quirement. This allows for a reduced total solution cost and footprint area. applicaTions inForMaTion Forexample,inanFPGAcorevoltageapplication,fora12V input, 0.9V output at 72A design, a total overall ±3% total voltage regulation window is required in responding to a 25% load step transient. Figure 3 illustrates the benefit of overall output capacitor reduction versus improved total DC accuracy by using 100µF ceramic output capacitors. Figure 2. Typical Load Step Transient Response with DC Voltage Accuracy Variance ALLOWABLE REGULATION WINDOW DC ACCURACY LOAD STEP AC OVERSHOOT AC UNDERSHOOT 46501 F02 TOTAL DC ACCURACY (%) 5000 4000 3000 2000 1000 4500 3500 2500 1500 500 0 0.8 1.5 46501 F03 2.0 1.2 2200 3200 2600 4700 Figure 3. Overall Output Capacitor vs Total DC Accuracy VIN to VOUT Step-Down Ratios There are restrictions in the maximum VIN and VOUT step- down ratio that can be achieved for a given input voltage. Each output of the LTM4650-1 is capable of 98% duty cycle, but the VIN to VOUT minimum dropout is still shown as a function of its load current and will limit output cur- rent capability related to high duty cycle on the top side switch.Minimumon-timetON(MIN)isanotherconsideration in operating at a specified duty cycle while operating at a certain frequency due to the fact that tON(MIN) < D/fSW, where D is duty cycle and fSW is the switching frequency. tON(MIN) is specified in the electrical parameters as 90ns. Output Voltage Programming ThePWMcontrollerhasaninternal0.6Vreferencevoltage. AsshownintheBlockDiagram,a60.4kΩinternalfeedback resistor connects between the VOUTS1 to VFB1 and VOUTS2 to VFB2. It is very important that these pins be connected to their respective outputs for proper feedback regulation. Overvoltage can occur if these VOUTS1andVOUTS2pinsare left floating when used as individual regulators, or at least one of them is used in paralleled regulators. The output voltage will default to 0.6V with no feedback resistor on |
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