Datasheet LTM8049 (Analog Devices) - 10

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OPERATION
The LTM8049 contains two stand-alone switching DC/ The LTM8049 also features RUN and SS pins to control DC converters; either one may be configured as a SEPIC the start-up behavior of the device. The RUN pin may also (single-ended primary inductance converter) or inverting be used to implement an accurate undervoltage lockout power supply simply by tying VOUTN or VOUTP to GND, function by applying a resistor network to the RUN pin. respectively. It accepts an input voltage up to 20VDC. The The LTM8049 features frequency foldback to protect the output is adjustable between 2.5V and 24V for the SEPIC, power switches during a fault or output current overload. and between –2.5V and –24V for the inverting configura- During start-up, frequency foldback also limits the current tion. The LTM8049 can provide 1.5A at VIN = 12V when the LTM8049 delivers to the load. The user must evaluate VOUT = 5V or –5V at ambient room temperature. the start-up behavior of the LTM8049 to ensure that it As shown in the Block Diagram, the LTM8049 contains a properly powers up the load. current mode control er, power switching element, power The LTM8049 is equipped with a thermal shutdown to coupled inductor, power Schottky diode and a modest protect the device during momentary overload conditions. amount of input and output capacitance. The LTM8049 It is set above the 125°C absolute maximum internal tem- is a fixed frequency PWM regulator. perature rating to avoid interfering with normal specified The LTM8049 switching can free run by applying a resis- operation, so internal device temperatures will exceed the tor to the RT pin or synchronize to an external source at absolute maximum rating when the overtemperature pro- a frequency between 200kHz and 2.5MHz. To synchronize tection is active. Therefore, continuous or repeated activa- to an external source, drive a valid signal source into the tion of the thermal shutdown may impair device reliability. SYNC pin. See Synchronization in the Applications Section for more details.
APPLICATIONS INFORMATION
For most applications, the design process is straight for- allowed to switch is given in Table 1 in the fMAX column, ward, summarized as follows: while the recommended frequency (and RT value) for opti- 1. Look at Table 1 and find the row that has the desired mal efficiency over the given input condition is given in input range and output voltage. the fOPTIMAL column. Running the LTM8049 faster than the recommended frequency may reduce the usable input 2. Apply the recommended CIN, COUT, RADJ and RT voltage range. values. While these component combinations have been tested
Capacitor Selection Considerations
for proper operation, it is incumbent upon the user to The CIN and COUT capacitor values in Table 1 are the mini- verify proper operation over the intended system’s line, mum recommended values for the associated operating load and environmental conditions. Bear in mind that the conditions. Applying capacitor values below those indi- maximum output current is limited by junction tempera- cated in Table 1 is not recommended, and may result in ture, the relationship between the input and output voltage undesirable operation. Using larger values is generally magnitude and polarity and other factors. Please refer acceptable, and can yield improved dynamic response, to the graphs in the Typical Performance Characteristics if it is necessary. Again, it is incumbent upon the user to section for guidance. verify proper operation over the intended system’s line, Table 1 gives the recommended component values and load and environmental conditions. configuration for a single channel. Each channel may be Ceramic capacitors are small, robust and have very low configured independently. The maximum frequency (and ESR. However, not all ceramic capacitors are suitable. X5R attendant RT value) at which the LTM8049 should be and X7R types are stable over temperature and applied Rev B 10 For more information www.analog.com 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 Photo Revision History Typical Application Related Parts