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LTC3605 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3605 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 22 page  LTC3605 10 3605fd For more information www.linear.com/LTC3605 Minimum Off-Time and Minimum On-Time Considerations The minimum off-time, tOFF(MIN), is the smallest amount of time that the LTC3605 is capable of turning on the bot- tom power MOSFET, tripping the current comparator and turning the power MOSFET back off. This time is generally about 70ns. The minimum off-time limit imposes a maxi- mum duty cycle of tON/(tON + tOFF(MIN)). If the maximum duty cycle is reached, due to a dropping input voltage for example, then the output will drop out of regulation. The minimum input voltage to avoid dropout is: VIN(MIN) = VOUT • tON + tOFF(MIN) tON Conversely, the minimum on-time is the smallest dura- tion of time in which the top power MOSFET can be in its “on” state. This time is typically 40ns. In continuous mode operation, the minimum on-time limit imposes a minimum duty cycle of: DCMIN = f • tON(MIN) where tON(MIN) is the minimum on-time. As the equation shows, reducing the operating frequency will alleviate the minimum duty cycle constraint. In the rare cases where the minimum duty cycle is surpassed, the output voltage will still remain in regula- tion, but the switching frequency will decrease from its programmed value. This is an acceptable result in many applications, so this constraint may not be of critical importance in most cases. High switching frequencies may be used in the design without any fear of severe consequences. As the sections on inductor and capacitor selection show, high switching frequencies allow the use of smaller board components, thus reducing the size of the application circuit. CIN and COUT Selection The input capacitance, CIN, is needed to filter the trapezoi- dal wave current at the drain of the top power MOSFET. To prevent large voltage transients from occurring, a low ESR input capacitor sized for the maximum RMS current should be used. The maximum RMS current is given by: IRMS ≅IOUT(MAX) VOUT VIN VIN VOUT – 1 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 deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. For low input voltage applications, sufficient bulk input capacitance is needed to minimize transient effects during output load changes. The selection of COUT is determined by the effective series resistance(ESR)thatisrequiredtominimizevoltageripple and load step transients as well as the amount of bulk capacitance that is necessary to ensure that the control loop is stable. Loop stability can be checked by viewing the load transient response. The output ripple, DVOUT, is determined by: DVOUT < DIL 1 8 • f • COUT +ESR ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ The output ripple is highest at maximum input voltage since DIL increases with input voltage. Multiple capaci- tors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special polymer, aluminum electrolytic, and ceramic capacitors are all available in surface mount packages. Special polymer capacitors are very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important to only use types that have been surge tested foruseinswitchingpowersupplies.Aluminumelectrolytic capacitors have significantly higher ESR, but can be used in cost-sensitive applications provided that consideration is given to ripple current ratings and long-term reliability. Ceramic capacitors have excellent low ESR characteristics and small footprints. Their relatively low value of bulk capacitance may require multiples in parallel. operaTion |
Similar Part No. - LTC3605_15 |
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Similar Description - LTC3605_15 |
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