Datasheet LT3512 (Analog Devices) - 7

LT3512
operaTion
The LT3512 is a current mode switching regulator IC de- controls the current level just as it does in normal current signed specifically for the isolated flyback topology. The mode operation, but instead of turning the switch on at the key problem in isolated topologies is how to communicate start of the oscillator period, the part turns on the switch information regarding the output voltage from the isolated when the secondary-side winding current is zero. secondary side of the transformer to the primary side. Historically, optoisolators or extra transformer windings
Boundary Mode Operation
communicate this information across the transformer. Boundary mode is a variable frequency, current mode Optoisolator circuits waste output power, and the extra switching scheme. The switch turns on and the inductor components increase the cost and physical size of the current increases until a VC pin controlled current limit. power supply. Optoisolators can also exhibit trouble due After the switch turns off, the voltage on the SW pin rises to limited dynamic response, nonlinearity, unit-to-unit to the output voltage divided by the secondary-to-primary variation and aging over life. Circuits employing an extra transformer turns ratio plus the input voltage. When the transformer winding also exhibit deficiencies. Using an secondary current through the diode falls to zero, the SW extra winding adds to the transformer’s physical size and pin voltage falls below V cost, and dynamic response is often mediocre. IN. A discontinuous conduction mode (DCM) comparator detects this event and turns the In the LT3512, the primary-side flyback pulse provides switch back on. information about the isolated output voltage. In this man- Boundary mode returns the secondary current to zero every ner, neither optoisolator nor extra transformer winding is cycle, so parasitic resistive voltage drops do not cause load required for regulation. Two resistors program the output regulation errors. Boundary mode also allows the use of a voltage. Since this IC operates in boundary mode, the part smaller transformer compared to continuous conduction calculates output voltage from the switch pin when the mode and does not exhibit subharmonic oscillation. secondary current is almost zero. At low output currents, the LT3512 delays turning on the The Block Diagram shows an overall view of the system. switch, and thus operates in discontinuous mode. Unlike Many of the blocks are similar to those found in traditional traditional flyback converters, the switch has to turn on switching regulators including internal bias regulator, os- to update the output voltage information. Below 0.6V on cillator, logic, current amplifier, current comparator, driver, the VC pin, the current comparator level decreases to and output switch. The novel sections include a special its minimum value, and the internal oscillator frequency flyback error amplifier and a temperature compensation decreases. With the decrease of the internal oscillator, circuit. In addition, the logic system contains additional the part starts to operate in DCM. The output current is logic for boundary mode operation. able to decrease while still allowing a minimum switch off The LT3512 features boundary mode control, where the part time for the flyback error amplifier. The typical minimum operates at the boundary between continuous conduction internal oscillator frequency with VC equal to 0V is 40kHz. mode and discontinuous conduction mode. The VC pin 3512fb 7 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