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LT3509EMSE-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LT3509EMSE-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT3509 11 3509f APPLICATIONS INFORMATION In any step-down switcher the duty cycle when operating in continuous, or fixed frequency, mode is dependent on the step-down ratio. This is because for a constant average load current the decay of the inductor current when the switch is off must match the increase in inductor current when the switch is on. The can be estimated by the following formula: DC VV VV V OUT F IN SW F = + −+ Where: DC = Duty Cycle (Fraction of Cycle when Switch is On) VOUT = Output Voltage VIN = Input Voltage VF = Catch Diode Forward Voltage VSW = Switch Voltage Drop Note: This formula neglects switching and inductor losses so in practice the duty cycle may be slightly higher. It is clear from this equation that the duty cycle will approach 100% as the input voltage is reduced and become smaller as the input voltage increases. There are practical limits to the minimum and maximum duty cycles for continuous operation due to the switch minimum off and on times. These are independent of operating frequency so it is clear that range of usable duty cycle is inverserly proportional to frequency. Therefore at higher frequency the input voltage range (for constant frequncy operation) will narrow. The minimum duty cycle is given by: DC f t MIN SW ON MIN = • () Where: fSW = Switching Frequency tMINON = Switch Minimum on Time The minimum on time increases with increasing tempera- ture so the value for the maximum operating temperature should be used. See the Minimum ON Time vs Load graph in the Typical Performance Characteristics. The maximum input voltage for this duty cycle is given by: V VV DC VV IN MAX OUT F MIN FSW () = + −+ Above this voltage the only way the LT3509 can maintain regulation is to skip cycles so the effective freqeuncy will reduce. This will cause an increase in ripple and the switch- ing noise will shift to a lower frequency. This calculation will in practice drive the maximum switching frequency for a desired step-down ratio. Figure 3. Continuous Mode IL 0.5A/DIV VOUT 100mV/DIV (AC COUPLED) TIME 1μs/DIV 3509 F03 Figure 4. Pulse Skipping IL 0.5A/DIV VOUT 100mV/DIV (AC COUPLED) TIME 1μs/DIV 3509 F04 |
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