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LTC3705 数据表(PDF) 10 Page - Linear Technology |
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LTC3705 数据表(HTML) 10 Page - Linear Technology |
10 / 20 page LTC3705 10 3705fb UVLO The UVLO pin is connected to a resistive voltage divider connected to VIN as shown in Figure 2. The voltage threshold on the UVLO pin for VIN rising is 1.242V. To introduce hysteresis, the LTC3705 draws 4.9 µA from the UVLO pin when VIN is rising. The hysteresis is therefore user adjustable and depends on the value of R1. The UVLO threshold for VIN rising is: VV RR R RA IN UVLO RISING (, ) (. ) ( . ) = + +µ 1 242 12 2 14 9 the volt-second limit circuit. The duty cycle is always limited to 50% 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 LTC3705 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 top switch, the transformer primary, and the bottom switch is sensed by a resistor connected between the source of the bottom switch and GND. If the voltage across this resistor exceeds 300mV, the LTC3705 initiates a fault. Bootstrap Refresh The LTC3705 incorporates a unique bootstrap refresh circuit to ensure that the bootstrap supply (BOOST) for the top switch has adequate voltage for operation at low duty cycles. Therefore, the LTC3705 does not require a undervoltage lockout for the bootstrap supply and a po- tential source of unexpected shutdowns is eliminated. Voltage Feedforward The LTC3705 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 50% 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 takes 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 50% 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 LTC3705 is used in standalone primary-side mode, feedback is provided by an optoisolator connected to the FB/IN+ pin. The LTC3705 has a built optoisolator bias circuit which eliminates the need for external components. OPERATIO APPLICATIO S I FOR ATIO The LTC3705 also has 16mV of voltage hysteresis on the UVLO pin so that the UVLO threshold for VIN falling is: VV RR R IN UVLO FALLING (, ) (. ) = + 1 226 12 2 To implement external Run/Stop control, connect a small NMOS to the UVLO pin as shown in Figure 2. Turning the NMOS on grounds the UVLO pin and prevents the LTC3705 from running. |
类似零件编号 - LTC3705 |
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类似说明 - LTC3705 |
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