Datasheet LT3972 (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | 33V, 3.5A, 2.4MHz Step-Down Switching Regulator with 75μA Quiescent Current |
| Pages / Page | 24 / 8 — OPERATION |
| File Format / Size | PDF / 346 Kb |
| Document Language | English |
OPERATION
Model Line for this Datasheet
Text Version of Document
LT3972
OPERATION
The LT3972 is a constant frequency, current mode step- to generate a voltage at the BOOST pin that is higher than down regulator. An oscillator, with frequency set by RT, the input supply. This allows the driver to fully saturate enables an RS fl ip-fl op, turning on the internal power the internal bipolar NPN power switch for effi cient switch. An amplifi er and comparator monitor the current operation. fl owing between the VIN and SW pins, turning the switch To further optimize effi ciency, the LT3972 automatically off when this current reaches a level determined by the switches to Burst Mode operation in light load situations. voltage at VC. An error amplifi er measures the output Between bursts, all circuitry associated with controlling voltage through an external resistor divider tied to the FB the output switch is shut down, reducing the input supply pin and servos the VC pin. If the error amplifi er’s output current to 75μA in a typical application. increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the The oscillator reduces the LT3972’s operating frequency V when the voltage at the FB pin is low. This frequency C pin provides current limit. The VC pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented foldback helps to control the output current during start- by generating a voltage ramp at the RUN/SS pin using an up and overload. external resistor and capacitor. The LT3972 contains a power good comparator which trips An internal regulator provides power to the control circuitry. when the FB pin is at 91% of its regulated value. The PG The bias regulator normally draws power from the V output is an open-collector transistor that is off when the IN pin, but if the BD pin is connected to an external voltage higher output is in regulation, allowing an external resistor to pull than 3V bias power will be drawn from the external source the PG pin high. Power good is valid when the LT3972 is (typically the regulated output voltage). This improves enabled and VIN is above 3.6V. effi ciency. The RUN/SS pin is used to place the LT3972 The LT3972 has an overvoltage protection feature which in shutdown, disconnecting the output and reducing the disables switching action when the VIN goes above 35V input current to less than 0.5μA. typical (33V minimum). When switching is disabled, the The switch driver operates from either the input or from LT3972 can safely sustain input voltages up to 62V. the BOOST pin. An external capacitor and diode are used 3972fa 8
OPERATION
The LT3972 is a constant frequency, current mode step- to generate a voltage at the BOOST pin that is higher than down regulator. An oscillator, with frequency set by RT, the input supply. This allows the driver to fully saturate enables an RS fl ip-fl op, turning on the internal power the internal bipolar NPN power switch for effi cient switch. An amplifi er and comparator monitor the current operation. fl owing between the VIN and SW pins, turning the switch To further optimize effi ciency, the LT3972 automatically off when this current reaches a level determined by the switches to Burst Mode operation in light load situations. voltage at VC. An error amplifi er measures the output Between bursts, all circuitry associated with controlling voltage through an external resistor divider tied to the FB the output switch is shut down, reducing the input supply pin and servos the VC pin. If the error amplifi er’s output current to 75μA in a typical application. increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the The oscillator reduces the LT3972’s operating frequency V when the voltage at the FB pin is low. This frequency C pin provides current limit. The VC pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented foldback helps to control the output current during start- by generating a voltage ramp at the RUN/SS pin using an up and overload. external resistor and capacitor. The LT3972 contains a power good comparator which trips An internal regulator provides power to the control circuitry. when the FB pin is at 91% of its regulated value. The PG The bias regulator normally draws power from the V output is an open-collector transistor that is off when the IN pin, but if the BD pin is connected to an external voltage higher output is in regulation, allowing an external resistor to pull than 3V bias power will be drawn from the external source the PG pin high. Power good is valid when the LT3972 is (typically the regulated output voltage). This improves enabled and VIN is above 3.6V. effi ciency. The RUN/SS pin is used to place the LT3972 The LT3972 has an overvoltage protection feature which in shutdown, disconnecting the output and reducing the disables switching action when the VIN goes above 35V input current to less than 0.5μA. typical (33V minimum). When switching is disabled, the The switch driver operates from either the input or from LT3972 can safely sustain input voltages up to 62V. the BOOST pin. An external capacitor and diode are used 3972fa 8
