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LCLV Datasheet(PDF) 9 Page - Linear Technology |
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LCLV Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LT3502/LT3502A 9 3502fc The LT3502/LT3502A are constant frequency, current mode step-down regulators. An oscillator enables an RS flip-flop, turning on the internal 500mA 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 VC. 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. The SHDN pin voltage during soft-start also reduces the oscillator frequency to avoid hitting current limit during start-up. An internal regulator provides power to the control cir- cuitry. This regulator includes an undervoltage lockout to prevent switching when VIN is less than ~3V. The SHDN pin is used to place the LT3502/LT3502A in shutdown, disconnecting the output and reducing the input current to less than 2μA. The switch driver operates from either VIN or from the BOOST pin. An external capacitor and the internal 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 operation. A comparator monitors the current flowing through the catch diode via the DA pin and reduces the LT3502/ LT3502A’s operating frequency when the DA pin current exceeds the 650mA valley current limit. This frequency foldback helps to control the output current in fault conditions such as shorted output with high input volt- age. The DA comparator works in conjunction with the switch peak current limit comparator to determine the maximum deliverable current of the LT3502/LT3502A. The peak current limit comparator is used in normal current mode operations and is used to turn off the switch. The DA valley current comparator monitors the catch diode current and will delay switching until the catch diode current is below the 650mA limit. Maximum deliverable current to the output is therefore limited by both switch peak current limit and DA valley current limit. OPERATION |
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