Datasheet LTM8023 (Analog Devices) - 9

LTM8023
OPERATION
The LTM8023 is a standalone nonisolated step-down voltage higher than 2.8V, bias power will be drawn from switching DC/DC power supply. It can deliver up to 2A of the external source (typically the regulated output volt- DC output current with only bulk external input and output age). This improves efficiency. The RUN/SS pin is used capacitors. This module provides a precisely regulated to place the LTM8023 in shutdown, disconnecting the output voltage programmable via one external resistor output and reducing the input current to less than 1µA. from 0.8VDC to 10VDC. The input voltage range is 3.6V to To further optimize efficiency, the LTM8023 automatically 36V. Given that the LTM8023 is a step-down converter, switches to Burst Mode operation in light load situations. make sure that the input voltage is high enough to support Between bursts, all circuitry associated with controlling the desired output voltage and load current. A simplified the output switch is shut down reducing the input supply Block Diagram is given on the previous page. current to 50µA in a typical application. The oscillator The LTM8023 contains a current mode controller, power reduces the LTM8023’s operating frequency when the switching element, power inductor, power Schottky diode voltage at the ADJ pin is low. This frequency foldback helps and a modest amount of input and output capacitance. to control the output current during start-up and overload. The LTM8023 is a fixed frequency PWM regulator. The The LTM8023 contains a power good comparator which switching frequency is set by simply connecting the trips when the ADJ pin is at 92% of its regulated value. appropriate resistor value from the RT pin to GND. The PGOOD output is an open-collector transistor that is An internal regulator provides power to the control cir- off when the output is in regulation, allowing an external cuitry. The bias regulator normally draws power from the resistor to pull the PGOOD pin high. Power good is valid V when the LTM8023 is enabled and VIN is above 3.6V. IN pin, but if the BIAS pin is connected to an external
APPLICATIONS INFORMATION
For most applications, the design process is straight indicated in Table 1 is not recommended, and may result forward, summarized as follows: in undesirable operation. Using larger values is generally 1. Look at Table 1 and find the row that has the desired acceptable, and can yield improved dynamic response, if input range and output voltage. it is necessary. Again, it is incumbent upon the user to verify proper operation over the intended system’s line, 2. Apply the recommended CIN, COUT, RADJ and RT values. load and environmental conditions. 3. Connect BIAS as indicated. Ceramic capacitors are small, robust and have very low While these component combinations have been tested ESR. However, not all ceramic capacitors are suitable. for proper operation, it is incumbent upon the user to X5R and X7R types are stable over temperature and ap- verify proper operation over the intended system’s line, plied voltage and give dependable service. Other types, load and environmental conditions. including Y5V and Z5U have very large temperature and voltage coefficients of capacitance. In an application cir-
Capacitor Selection Considerations
cuit they may have only a small fraction of their nominal capacitance resulting in much higher output voltage ripple The CIN and COUT capacitor values in Table 1 are the than expected. minimum recommended values for the associated oper- ating conditions. Applying capacitor values below those 8023fj For more information www.linear.com/LTM8023 9