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LTM4634 Arkusz danych(PDF) 11 Page - Linear Technology |
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LTM4634 Arkusz danych(HTML) 11 Page - Linear Technology |
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11 / 32 page  LTM4634 11 4634f For more information www.linear.com/LTM4634 applicaTions inForMaTion The typical LTM4634 application circuit is shown in Fig- ure 22. External component selection is primarily deter- mined by the maximum load current and output voltage. RefertoTable4forspecificexternalcapacitorrequirements for particular applications. VIN to VOUT Step-Down Ratios There are restrictions in the VIN to VOUT step-down ratio that can be achieved for a given input voltage. The VIN to VOUT minimum dropout is a function of load current and at very low input voltage and high duty cycle applications output power may be limited as the internal top power MOSFET is not rated for 5A operation at higher ambient temperatures. At very low duty cycles the minimum 100ns on-time must be maintained. See the Frequency Adjust- ment section and temperature derating curves. Output Voltage Programming The PWM controller has an internal 0.8V ±1% reference voltage. As shown in the Block Diagram, a 60.4k preci- sion internal feedback resistor connects the VOUT and VFB pins together. The output voltage will default to 0.8V with no feedback resistor. Adding a resistor RFB from VFB to ground pro- grams the output voltage: VOUT = 0.8V • 60.4k +RFB RFB or RFB = 48.32k VOUT –0.8 Table 1. VFB Resistor Table vs Various Output Voltages VOUT(V) 0.8 1.0 1.2 1.5 1.8 2.5 3.3 5.0 12.0 RFB (kΩ) Open 243 121 69.8 48.7 28.7 19.1 11.5 4.32 In the parallel operation the following pins should be tied together, VFB1 and VFB2 pins, COMP1 and COMP2 pins, TK/SS1 and TK/SS2, and RUN1 and RUN2. For parallel operation of VOUT1 and VOUT2, connect VFB1 and VFB2 together with a single resistor to ground whose value is determined by: RFB = 60.4k 2 VOUT 0.8 –1 Input Capacitors The LTM4634 module should be connected to a low AC impedance DC source. Additional input capacitors are neededfortheRMSinputripplecurrentrating.TheICIN(RMS) equation which follows can be used to calculate the input capacitor requirement for each channel. Typically 4.7µF to 10µF X7R ceramics are a good choice with RMS ripple currentratingsof~2Aeach.A47µFto100µFsurfacemount aluminumelectrolyticcapacitorcanbeusedformoreinput bulk capacitance. This bulk input capacitor is only needed if the input source impedance is compromised by long inductive leads, traces or not enough source capacitance. If low impedance power planes are used, then this bulk capacitor is not needed. For a buck converter, the switching duty cycle can be estimated as: D = VOUT VIN Without considering the inductor ripple current, for each output, the RMS current of the input capacitor can be estimated as: ICIN(RMS) = IOUT(MAX) η% • D• 1–D ( ) (1) In the previous equation, η% is the estimated efficiency of the power module in decimal form (0.nn) for a given VOUT-to-VIN ratio. The selection of CIN is simplified by the 3-phase architec- ture and its impact on the worst-case RMS current draw occurs when only one channel is operating. This is true when the three channels are powered from a common VIN. The channel with the highest duty cycle D peaking at 0.5 and maximum load current needs to be used in the above formula. This will give the maximum RMS capacitor current requirement. Increasing the output current drawn from the other channels will actually decrease the input RMS ripple current from its maximum value. The out-of- phase technique typically reduces the input capacitor’s RMS ripple current by a factor of 50% when compared to a single phase power supply solution. If the three channels are powered from independent input sources, then each |
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