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LTM4651 Datasheet(PDF) 18 Page - Linear Technology |
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LTM4651 Datasheet(HTML) 18 Page - Linear Technology |
18 / 34 page LTM4651 18 4651f For more information www.linear.com/LTM4651 Hot-Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypasscapacitors(CDandCINH)oftheLTM4651.However, these capacitors can cause problems if the LTM4651 is plugged into a live supply (see Linear Technology Ap- plication Note 88 for a complete discussion). The low loss ceramic capacitor combined with stray inductance in series with the power source forms an under damped tank circuit, and the voltage at the VIN pin of the LTM4651 can ring to twice the nominal input voltage, possibly ex- ceeding the LTM4651’s rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LTM4651 into an energized supply, the input network should be designed to prevent this overshoot by introducing a damping element into the path of current flow. This is often done by adding an inexpensive elec- trolytic bulk capacitor (CINL) across the input terminals of the LTM4651. The selection criteria for CINL calls for: an ESR high enough to damp the ringing; a capacitance value several times larger than CINH. CINL does not need to be located physically close to the LTM4651; it should be located close to the application board’s input connec- tor, instead. INTVCC and EXTVCC Connection When RUN is logic high, an internal low dropout regula- tor regulates an internal supply, INTVCC, that powers the control circuitry for driving LTM4651’s internal MOSFETs. INTVCC is regulated at 3.3V above VOUT–. In this manner, the LTM4651’s INTVCC is directly powered from SVIN, by default. The gate driver current through the LDO is about 20mA for a typical 1MHz application. The internal LDO power dissipation can be calculated as: PLDO_LOSS(INTVCC) =20mA •(SVIN+|VOUT– |–3.3V) (17) The LDO draws current off of EXTVCCinsteadofSVINwhen EXTVCCistiedtoavoltagehigherthan3.2VaboveVOUT–and SVIN is 5V above VOUT–. For output voltages at or below –4V, this pin can be connected to PGND through an RC- filter. When the internal LDO derives power from EXTVCC instead of SVIN, the internal LDO power dissipation is: PLDO_LOSS(EXTVCC) =20mA •(|VOUT– |–3V) (18) TherecommendedvalueoftheresistorbetweenPGNDand EXTVCC is roughly |VOUT–| • 4Ω/V. This resistor, REXTVCC, must be rated to continually dissipate (0.02A)² • REXTVCC. The primary purpose of this resistor is to prevent EXTVCC overstress under a fault condition. For example, when an inductive short-circuit is applied to the module’s output, VOUT– may be briefly dragged above EXTVCC— forward- biasing the VOUT–-to-EXTVCC body diode. This resistor limits the magnitude of current flow into EXTVCC. Bypass EXTVCC to VOUT– with 1μF of X5R (or better) MLCC. Multiphase Operation Multiple LTM4651 devices can be paralleled for higher output current applications. For lowest input and output voltage and current ripples, it is advisable to synchronize paralleled LTM4651s to an external clock (within ±40% of the target switching frequency set by fSET—see Test Circuit 1). See Figure 34 for an example of a synchroniz- ing circuit. LTM4651modulescanbeparalleledwithoutsynchronizing circuits: just be aware that some beat-frequency ripple will be present in the output voltage and reflected input current by virtue of the fact that such modules are not operating at identical, synchronized switching frequencies. The LTM4651 device is an inherently current mode con- trolled device, so parallel modules will have good current sharing’s shown in Figure 35. This helps balance the thermals on the design. To parallel LTM4651s, connect the respective COMPa, ISETa, and GNDSNS pins of each LTM4651 together to sharethecurrentevenly.Inaddition,tietherespectiveRUN pins of paralleled LTM4651 devices together, to ensure proper start-up and shutdown behavior. Figure 34 shows a schematic of LTM4651 devices operating in parallel. Note that for parallel applications, EQ. 8 becomes: RSET = –VOUT– 50µA •N (19) APPLICATIONS INFORMATION |
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