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LTM4639 Folha de dados(PDF) 10 Page - Linear Technology |
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LTM4639 Folha de dados(HTML) 10 Page - Linear Technology |
10 / 28 page LTM4632 10 4632fc For more information www.linear.com/LTM4632 APPLICATIONS INFORMATION Channel 1 Output Voltage Programming (Configured as VDDQ) The PWM controller for the VOUT1 has an internal 0.6V reference voltage. As shown in the Block Diagram, a 60.4k internal feedback resistor connects VOUT1 and FB1 pins together. Adding a resistor RFB from FB1 pin to GND pro- grams the output voltage: RFB = 0.6V VOUT – 0.6V • 60.4k Table 1. VFB Resistor Table (1%) vs Various Output Voltages VOUT(V) 0.6 1.0 1.2 1.3 1.5 1.8 2.5 RFB(k) OPEN 90.9 60.4 52.3 40.2 30.1 19.1 Channel 2 Output Voltage Programming (Configured as VTT) The PWM controller for the VOUT2 uses VTTR voltage as a reference voltage. VOUT2 is directly connected to the negative side of the error compiler to internally program VOUT2 to equal to VTTR voltage, which equals to one half of VDDQIN voltage. VOUT2 = VTTR = VDDQIN/2 In a complete DDR memory power application which require both VDDQ supply and VTT terminal outputs, configure LTM4632 Channel 1 as VDDQ output by add- ing a feed-back resistor from FB1 pin to GND. Feed VOUT1 (VDDQ output) voltage to VDDQIN pin to program Channel 2 as VTT output which equals half of the Channel 1 (VDDQ output) voltage. Input Decoupling Capacitors The LTM4632 module should be connected to a low AC-impedance DC source. For each regulator channel, one piece 4.7µF input ceramic capacitor is required for RMS ripple current decoupling. Bulk input capacitor is only needed when the input source impedance is com- promised by long inductive leads, traces or not enough source capacitance. The bulk capacitor can be an electro- lytic aluminum capacitor and polymer capacitor. Without considering the inductor current ripple, for each output, the RMS current of the input capacitor can be estimated as: ICIN(RMS) = IOUT(MAX) η% • D •(1– D) where η% is the estimated efficiency of the power module. Output Decoupling Capacitors With an optimized high frequency, high bandwidth design, only single piece of 22µF low ESR output ceramic capaci- tor is required for each LTM4632 output to achieve low output voltage ripple and very good transient response. Additional output filtering may be required by the sys- tem designer, if further reduction of output ripples or dynamic transient spikes is required. Table 5 shows a matrix of different output voltages and output capacitors to minimize the voltage droop and overshoot during a 0.75A (25%) load step transient. Multiphase operation will reduce effective output ripple as a function of the number of phases. Application Note 77 discusses this noise reduction versus output ripple current cancella- tion, but the output capacitance will be more a function of stability and transient response. The Linear Technology LTpowerCAD Design Tool is available to download online for output ripple, stability and transient response analysis and calculating the output ripple reduction as the number of phases implemented increases by N times. Burst Mode Operation In applications where high efficiency at intermediate current are more important than output voltage ripple, burst mode operation could be used on Channel 1 by connecting SYNC/MODE pin to INTVCC to improve light load efficiency. In Burst Mode operation, a current rever- sal comparator (IREV) detects the negative inductor cur- rent and shuts off the bottom power MOSFET, resulting in discontinuous operation and increased efficiency. Both power MOSFETs will remain off and the output capacitor will supply the load current until the COMP voltage rises above the zero current level to initiate another cycle. |
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