Datasheet LT3511 (Analog Devices) - 7

LT3511
OPERATION
The LT3511 is a current mode switching regulator IC de- VC pin controls the current level just as it does in normal signed specifically for the isolated flyback topology. The current mode operation, but instead of turning the switch key problem in isolated topologies is how to communicate on at the start of the oscillator period, the part turns on the information regarding the output voltage from the isolated switch when the secondary-side winding current is zero. secondary side of the transformer to the primary side. Historically, opto-isolators or extra transformer windings
Boundary Mode Operation
communicate this information across the transformer. Boundary mode is a variable frequency, current mode Opto-isolator 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. Opto-isolators 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 VIN. A discontinuous conduction cost, and dynamic response is often mediocre. mode (DCM) comparator detects this event and turns the In the LT3511, 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 opto-isolator 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 LT3511 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 LT3511 features boundary mode control, where the time for the flyback error amplifier. The typical minimum part operates at the boundary between continuous con- internal oscillator frequency with VC equal to 0V is 40kHz. duction mode and discontinuous conduction mode. The 3511fc 7