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LTM4653 датащи(PDF) 14 Page - Linear Technology |
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LTM4653 датащи(HTML) 14 Page - Linear Technology |
14 / 32 page LTM4653 14 Rev 0 For more information www.analog.com where D (unitless) is the duty-cycle of MT, given by: D = VOUT VIN In rare cases where the minimum on-time restriction is violated, the frequency of the LTM4653 automatically and gradually folds back down to approximately one-fifth of its programmed switching frequency to allow VOUT to remain in regulation. See the Frequency Adjustment section. Be remindedofNotes2,3and5intheElectricalCharacteristics section regarding output current guidelines. Input Capacitors TheLTM4653achieveslowinputconductedEMInoisedue to tight layout and high-frequency bypassing of MOSFETs MT and MB within the module itself. A small filter inductor (400nH) is integrated in the input line (from VIN to VD), providing further noise attenuation—again, local to the switching MOSFETs. The VD and VIN pins are available for external input capacitors—CD and CINH—to form a high-frequency π filter. As shown in the Simplified Block Diagram, the ceramic capacitor CD on the LTM4653’s VD pins handles the majority of the RMS current into the DC/ DC converter power stage and requires careful selection, for that reason. See Figures 4 to 6 for demonstration of LTM4653’s EMI performance,meetingtheradiatedemissionsrequirements of EN55022B. The input capacitance, CD, is needed to filter the pulsed current drawn by MT. To prevent excessive voltage sag on VD, a low-effective series resistance (low-ESR, such as an X7R ceramic) input capacitor should be used, sized appropriately for the maximum CD RMS ripple current: ICD(RMS) = IOUT(MAX) η% • D • (1– D) where η%istheestimatedefficiencyofthepowermodule. (See Typical Performance Characteristics graphs.) Several capacitors may be paralleled to meet the applica- tion’s target size, height, and CDRMSripplecurrentrating. For lower input voltage applications, sufficient bulk input capacitance is needed to counteract line sag and transient OPERATION effects during output load changes. The bulk capacitor can be a switcher-rated aluminum electrolytic capacitor or a Polymer capacitor. Suggested values for CD and CINH are found in Table 7. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LTM4653’s VIN, SVIN,andVDpins.Aceramicinputcapacitorcombinedwith trace or cable inductance forms a high Q (underdamped) tank circuit. If the LTM4653 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the device’s rating. This situa- tion is easily avoided; see the Hot-Plugging Safely section. Output Capacitors Output capacitors COUTH and COUTL are applied to VOUT of theLTM4653.SufficientcapacitanceandlowESRarecalled for, to meet the output voltage ripple, loop stability, and transient requirements. COUTL can be a low ESR tantalum or polymer capacitor. COUTH is a ceramic capacitor. The typical output capacitance is 22μF (type X5R material, or better), if ceramic-only output capacitors are used. Table 7 shows a matrix of suggested output capacitors optimized for 2A transient step-loads applied at 2A/μs. Additional output filtering may be required by the system designer, if further reduction of output ripple or dynamic transient spike is required. The LTpowerCAD design tool is availablefortransientandstabilityanalysis.Stabilitycriteria are considered in the Table 7 matrix, and LTpowerCAD is available for stability analysis. Multiphase operation will reduce effective output ripple as a function of the num- ber of phases. Application Note 77 discusses this noise reduction versus output ripple current cancellation, but the output capacitance should be considered carefully as afunctionofstabilityandtransientresponse.LTpowerCAD can be used to calculate the output ripple reduction as the number of implemented phases increases by N times. External loop compensation can be applied to COMPa if needed, for transient response optimization. Forced Continuous Operation LeavetheCLKINpinopencircuittocommandtheLTM4653 for forced continuous operation. In this mode, the control loop is allowed to command the inductor peak current to |
Аналогичный номер детали - LTM4653 |
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Аналогичное описание - LTM4653 |
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