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LT3685IDD-PBF Datasheet(PDF) 8 Page - Linear Technology |
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LT3685IDD-PBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 24 page  LT3505 8 3505fc BLOCK DIAGRAM 7 Σ R DRIVER Q1 S OSC SLOPE COMP FREQUENCY FOLDBACK INT REG AND UVLO VC gm 780mV 3505 BD 5 3 4 Q Q 1 2 BOOST SW FB R2 R1 VOUT L1 D2 C3 C1 D1 VIN C2 VIN ON OFF GND 6 RT 8 VC C4 R3 SHDN OPERATION (Refer to Block Diagram) The LT3505 is a constant frequency, current mode step- downregulator.Aresistor-programmedoscillatorenables an RS flip-flop, turning on the internal 1.4A power switch Q1. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at the VC pin. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC node. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp (not shown) on the VC node provides current limit. The VC node is also clamped to the voltage on the SHDN pin; soft-start is implemented by generating a voltage ramp at the SHDN pin using an external resistor and capacitor. Aninternalregulatorprovidespowertothecontrolcircuitry. This regulator includes an undervoltage lockout to prevent switching when VIN is less than ~3.4V. The SHDN pin is used to place the LT3505 in shutdown, disconnecting the output and reducing the input current to less than 2µA. The switch driver operates from either the input or from the BOOST pin. An external capacitor and diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for efficient opera- tion. When the FB pin is low, the voltage at the RT pin decreases toreducetheoscillatorfrequency.Thisfrequencyfoldback helps to control the output current during start-up and overload. |
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Similar Description - LT3685IDD-PBF |
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