Datasheet LTM8031 (Analog Devices) - 8

LTM8031
operaTion
The LTM8031 is a standalone nonisolated step-down An internal regulator provides power to the control circuitry. switching DC/DC power supply. It can deliver up to 1A of The bias regulator can draw power from the VIN pin, but if DC output current with only bulk external input and output the BIAS pin is connected to an external voltage higher than capacitors. This module provides a precisely regulated 2.8V, bias power will be drawn from the external source output voltage programmable via one external resistor (typically the regulated output voltage). This improves from 0.8VDC to 10VDC. The input voltage range is 3.6V efficiency. The RUN/SS pin is used to place the LTM8031 to 36V. Given that the LTM8031 is a step-down converter, in shutdown, disconnecting the output and reducing the make sure that the input voltage is high enough to support input current to less than 1µA. the desired output voltage and load current. A simplified To further optimize efficiency, the LTM8031 automatically Block Diagram is given on the previous page. switches to Burst Mode operation in light load situations. The LTM8031 is designed with an input EMI filter and other Between bursts, all circuitry associated with controlling the features to make its radiated emissions compliant with output switch is shut down reducing the input supply cur- several EMC specifications including EN55022 class B. rent to 50µA in a typical application. The oscillator reduces Compliance with conducted emissions requirements may the LTM8031’s operating frequency when the voltage at the be obtained by adding a standard input filter. ADJ pin is low. This frequency foldback helps to control The LTM8031 contains a current mode controller, power the output current during start-up and overload. switching element, power inductor, power Schottky diode The LTM8031 contains a power good comparator which and a modest amount of input and output capacitance. The trips when the ADJ pin is at 90% of its regulated value. LTM8031 is a fixed frequency PWM regulator. The switch- The PGOOD output is an open-collector transistor that is ing frequency is set by simply connecting the appropriate off when the output is in regulation, allowing an external resistor value from the RT pin to GND. resistor to pull the PGOOD pin high. Power good is valid when the LTM8031 is enabled and VIN is above 3.6V.
applicaTions inForMaTion
For most applications, the design process is straight
Capacitor Selection Considerations
forward, summarized as follows: The CIN and COUT capacitor values in Table 1 are the 1. Look at Table 1 and find the row that has the desired minimum recommended values for the associated oper- input range and output voltage. ating conditions. Applying capacitor values below those 2. Apply the recommended C indicated in Table 1 is not recommended, and may result IN, COUT, RADJ and RT values. in undesirable operation. Using larger values is generally acceptable, and can yield improved dynamic response, if 3. Connect BIAS as indicated. it is necessary. Again, it is incumbent upon the user to As the integrated input EMI filter may ring in response to an verify proper operation over the intended system’s line, application of a step input voltage, a bulk capacitance, series load and environmental conditions. resistance or some clamping mechanism may be required. Ceramic capacitors are small, robust and have very low See the Hot-Plugging Safely section for details. ESR. However, not all ceramic capacitors are suitable. X5R While these component combinations have been tested for and X7R types are stable over temperature and applied proper operation, it is incumbent upon the user to verify voltage and give dependable service. Other types, includ- proper operation over the intended system’s line, load and ing Y5V and Z5U have very large temperature and voltage environmental conditions. coefficients of capacitance. In an application circuit they 8031fb 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical application Related Parts