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LTC3725IMSETR Datasheet(PDF) 10 Page - Linear Technology |
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LTC3725IMSETR Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTC3725 10 3725f exceeds 1.26V the gate drive is immediately turned “off.” Note that this is not considered a fault condition and the LTC3725 can run indefinitely with the switch duty cycle being determined by the volt-second limit circuit. The duty cycle is always limited to 75% to ensure that the power transformer flux always has time to reset before the start of the next cycle. In an alternate application, the volt-second limit can be used for open-loop regulation of the output against changes in VIN. Current Limit Current limit for the LTC3725 is principally a safety feature to protect the converter and is not part of a control function. The current that flows in series through the transformer primary and the switch is sensed by a resistor connected between the source of the switch and GND. If the voltage across this resistor exceeds 300mV, the LTC3725 initiates a fault. Voltage Feedforward The LTC3725 uses voltage feedforward to properly modu- late the duty cycle as a function of the input voltage. For secondary-side control with the LTC3706, voltage feedforward is used during start-up only. The duty cycle during start up is determined by comparison of the voltage on the SSFLT pin to a 75% duty cycle triangle wave with an amplitude of 2V. To implement voltage feedforward, the charging current for the soft-start capacitor is reduced in proportion to the input voltage. As a result, the initial rate of rise of the converter output voltage is held approxi- mately constant regardless of the input voltage. At some point during start-up, the LTC3706 begins to switch the pulse transformer and take over the soft-start. For operation with standalone primary-side control and optoisolator feedback, voltage feedforward is used during both start-up and normal operation. The duty cycle is determined by using a 75% duty cycle triangle wave with an amplitude equal to 66% of the voltage on the UVLO pin which is, in turn, proportional to VIN. The charging current for the soft-start capacitor is a constant 5.2 µA. During soft-start, the duty cycle is determined by comparing the voltage on the SSFLT pin to the triangle wave. Soft-start is concluded when the voltage on the SSFLT pin exceeds the voltage on the FB/IN+ pin. After the conclusion of soft- start, the duty cycle is determined by comparison of the voltage on the FB/IN+ pin to the triangle wave. Optoisolator Bias When the LTC3725 is used in standalone primary-side mode, feedback is provided by an optoisolator connected to the FB/IN+ pin. The LTC3725 has a built optoisolator bias circuit which eliminates the need for external components. OPERATIO |
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