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LTC1148-3.3 Datasheet(PDF) 10 Page - Linear Technology |
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LTC1148-3.3 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LTC1148 LTC1148-3.3/LTC1148-5 114835fd APPLICATIO S I FOR ATIO level threshold MOSFETs (VGS(TH) < 2.5V) are strongly recommended. The LTC1148/LTC1148HV series supply voltage must always be less than the absolute maximum VGS ratings for the MOSFETs. The maximum output current IMAX determines the RDS(ON) requirement for the two MOSFETs. When the LTC1148 series is operating in continuous mode, the simplifying assumption can be made that one of the two MOSFETs is always conducting the average load current. The duty cycles for the two MOSFETs are given by: P-Ch Duty Cycle = VOUT VIN N-Ch Duty Cycle = (VIN – VOUT) VIN From the duty cycles the required RDS(ON) for each MOS- FET can be derived: P-Ch RDS(ON) = VIN(PP) VOUT(IMAX2)(1 + δP) N-Ch RDS(ON) = VIN(PN) (VIN – VOUT)(IMAX2)(1 + δN) where PP and PN are the allowable power dissipations and dP and dN are the temperature dependencies of RDS(ON). PP and PN will be determined by efficiency and/or thermal requirements (see Efficiency Considerations). (1 + d) is generally given for a MOSFET in the form of a normalized RDS(ON) vs temperature curve, but d = 0.007/°C can be used as an approximation for low voltage MOSFETs. The Schottky diode D1 shown in Figure 1 only conducts during the dead-time between the conduction of the two power MOSFETs. D1’s sole purpose in life is to prevent the body diode of the N-channel MOSFET from turning on and storing charge during the dead time, which could cost as much as 1% in efficiency (although there are no other harmful effects if D1 is omitted). Therefore, D1 should be selected for a forward voltage of less than 0.7V when conducting IMAX. CIN and COUT Selection In continuous mode, the source of the P-channel MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: CIN Required IRMS ≈ IMAX [VOUT(VIN – VOUT)]1/2 VIN This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is com- monly used for design because even significant devia- tions do not offer much relief. Note that capacitor manufacturer’s ripple current ratings are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. Always consult the manufacturer if there is any question. An additional 0.1µF to 1µF ceramic capacitor is also required on VIN Pin 3 for high frequency decoupling. The selection of COUT is driven by the required effective series resistance (ESR). The ESR of COUT must be less than twice the value of RSENSE for proper operation of the LTC1148 series: COUT Required ESR < 2RSENSE Optimum efficiency is obtained by making the ESR equal to RSENSE. As the ESR is increased up to 2RSENSE, the efficiency degrades by less than 1%. If the ESR is greater than 2RSENSE, the voltage ripple on the output capacitor will prematurely trigger Burst Modeoperation, resulting in disruption of continuous mode and an efficiency hit which can be several percent. Manufacturers such as Nichicon and United Chemicon should be considered for high performance capacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR/size ratio of any aluminum electrolytic at a somewhat higher price. Once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. |
Similar Part No. - LTC1148-3.3_15 |
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Similar Description - LTC1148-3.3_15 |
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