Datasheet LTM8071 (Analog Devices) - 11

link to page 12 LTM8071
BLOCK DIAGRAM
VIN VIN BIAS CIN AUX CURRENT 2.2µH VOUT 0.2µF MODE CONTROLLER C 249k 10nF OUT 0.02µF GND FB RUN SHARE TR/SS SYNC RT PG 8071 BD VIN
OPERATION
The LTM8071 is a standalone nonisolated step-down The oscillator reduces the LTM8071’s operating frequency switching DC/DC power supply that can deliver 7.25A at when the voltage at the FB pin is low. This frequency fold- 3.3VOUT. The continuous current is determined by the back helps to control the output current during start-up internal operating temperature. It provides a precisely and overload. regulated output voltage programmable via one exter- The TR/SS node acts as an auxiliary input to the error nal resistor from 0.97V to 15V. The input voltage range amplifier. The voltage at FB servos to the TR/SS voltage is 3.6V to 60V. Given that the LTM8071 is a step-down until TR/SS exceeds about 1.6V. Soft-start is implemented converter, make sure that the input voltage is high enough by generating a voltage ramp at the TR/SS pin using an to support the desired output voltage and load current. A external capacitor which is charged by an internal con- simplified Block Diagram is given above. stant current. Alternatively, driving the TR/SS pin with The LTM8071 contains a current mode controller, power a signal source or resistive network provides a tracking switching elements, power inductor and a modest amount function. Do not drive the TR/SS pin with a low impedance of input and output capacitance. The LTM8071 is a fixed voltage source. See the Applications Information section frequency PWM regulator. The switching frequency is set for more details. by simply connecting the appropriate resistor value from The LTM8071 contains a power good comparator which the RT pin to GND. trips when the FB pin is between 0.89V and 1.05V, typical. An internal regulator provides power to the control cir- The PG output is an open-drain transistor that is off when cuitry. This bias regulator normally draws power from the the output is in regulation, allowing an external resistor VIN pin, but if the BIAS pin is connected to an external to pull the PG pin high. The PG signal is valid when VIN voltage higher than 3.2V, bias power is drawn from the is above 3.6V. If VIN is above 3.6V and RUN is low, PG external source (typically the regulated output voltage). will drive low. This improves efficiency. The RUN pin is used to place The LTM8071 is equipped with a thermal shutdown that the LTM8071 in shutdown, disconnecting the output and inhibits power switching at high junction temperatures. reducing the input current to a few μA. The activation threshold of this function is above 125°C to If SYNC is less than about 0.4V, the LTM8071 automati- avoid interfering with normal operation, so prolonged or cally switches to enhanced efficiency Burst Mode opera- repetitive operation under a condition in which the thermal tion in light or no load situations. Between bursts, all shutdown activates may damage or impair the reliability circuitry associated with controlling the output switch is of the device. shut down reducing the input supply current. Rev 0 For more information www.analog.com 11 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Package Photos Package Description Typical Application Related Parts