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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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LTM4636-1 데이터시트(PDF) 18 Page - Linear Technology |
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LTM4636-1 데이터시트(HTML) 18 Page - Linear Technology |
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18 / 38 page  LTM4636-1 18 46361fa For more information www.linear.com/LTM4636-1 APPLICATIONS INFORMATION Ratiometric tracking can be achieved by a few simple calculationsandtheslewratevalueappliedtothemaster’s TRACK/SS pin. As mentioned above, the TRACK/SS pin has a control range from 0V to 0.6V. The master’s TRACK/SS pin slew rate is directly equal to the master’s output slew rate in volts/time. The equation: MR SR • 4.99k RTB where MR is the master’s output slew rate and SR is the slave’s output slew rate in volts/time. When coincident tracking is desired, then MR and SR are equal, thus RTB is equal to 60.4k. RTA is derived from equation: RTA 0.6V VFB 4.99k VFB RFB1 – V TRACK RTB where VFB is the feedback voltage reference of the regula- tor, and VTRACK is 0.6V. Since RTB is equal to the 4.99k top feedback resistor of the slave regulator in equal slew rate or coincident tracking, then RTA is equal to RFB with VFB = VTRACK. Therefore RTB = 4.99k, and RTA = 4.99k in Figure 5. In ratiometric tracking, a different slew rate maybe desired for the slave regulator. RTB can be solved for when SR is slower than MR. Make sure that the slave supply slew rate is chosen to be fast enough so that the slave output voltage will reach its final value before the master output. For example, MR = 1.5V/ms, and SR = 1.2V/ms. Then RTB = 6.19k. Solve for RTA to equal 4.22k. For applications that do not require tracking or sequenc- ing, simply tie the TRACK/SS pin to INTVCC to let RUN control the turn on/off. When the RUN pin is below its threshold or the VIN undervoltage lockout, then TRACK/SS is pulled low. Default Overcurrent and Overvoltage Protection The LTM4636-1 has overcurrent protection (OCP) in a short circuit. The internal current comparator threshold folds back during a short to reduce the output current. An overvoltage condition (OVP) above 10% of the regulated outputvoltagewillforcethetopMOSFEToffandthebottom MOSFETonuntiltheconditioniscleared.Foldbackcurrent limiting is disabled during soft-start or tracking start-up. Temperature Monitoring Measuring the absolute temperature of a diode is pos- sible due to the relationship between current, voltage and temperature described by the classic diode equation: ID IS •e VD • VT or VD • VT •In ID IS where ID is the diode current, VD is the diode voltage, is the ideality factor (typically close to 1.0) and IS (satu- ration current) is a process dependent parameter. VT can be broken out to: VT k • T q where T is the diode junction temperature in Kelvin, q is the electron charge and k is Boltzmann’s constant. VT is approximately 26mV at room temperature (298K) and scales linearly with Kelvin temperature. It is this linear temperature relationship that makes diodes suitable tem- perature sensors. The IS term in the previous equation is the extrapolated current through a diode junction when the diode has zero volts across the terminals. The IS term varies from process to process, varies with temperature, and by definition must always be less than ID. Combining all of the constants into one term: KD •k q where KD = 8.62−5, and knowing ln(ID/IS) is always posi- tive because ID is always greater than IS, leaves us with the equation that: VD T KELVIN •KD •In ID IS |
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