Datasheet LT3575 (Analog Devices) - 7

LT3575
OPERATION
The LT3575 is a current mode switching regulator IC The LT3575 features a boundary mode control method, designed specifi cally for the isolated fl yback topology. The where the part operates at the boundary between continuous special problem normally encountered in such circuits is conduction mode and discontinuous conduction mode. The that information relating to the output voltage on the isolated VC pin controls the current level just as it does in normal secondary side of the transformer must be communicated to current mode operation, but instead of turning the switch the primary side in order to maintain regulation. Historically, on at the start of the oscillator period, the part detects this has been done with optoisolators or extra transformer when the secondary side winding current is zero. windings. Optoisolator circuits waste output power and the extra components increase the cost and physical size
Boundary Mode Operation
of the power supply. Optoisolators can also exhibit trouble Boundary mode is a variable frequency, current-mode due to limited dynamic response, nonlinearity, unit-to-unit switching scheme. The switch turns on and the inductor variation and aging over life. Circuits employing extra current increases until a VC pin controlled current limit. The transformer windings also exhibit defi ciencies. Using an voltage on the SW pin rises to the output voltage divided extra winding adds to the transformer’s physical size and by the secondary-to-primary transformer turns ratio plus cost, and dynamic response is often mediocre. the input voltage. When the secondary current through the diode falls to zero, the SW pin voltage falls below V The LT3575 derives its information about the isolated IN. A discontinuous conduction mode (DCM) comparator output voltage by examining the primary side fl yback detects this event and turns the switch back on. pulse waveform. In this manner, no optoisolator nor extra transformer winding is required for regulation. The output Boundary mode returns the secondary current to zero voltage is easily programmed with two resistors. Since this every cycle, so the parasitic resistive voltage drops do not IC operates in boundary control mode, the output voltage is cause load regulation errors. Boundary mode also allows calculated from the switch pin when the secondary current the use of a smaller transformer compared to continuous is almost zero. This method improves load regulation conduction mode and no subharmonic oscillation. without external resistors and capacitors. At low output currents the LT3575 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 a traditional fl yback converter, the switch has to turn on switching regulators including: internal bias regulator, to update the output voltage information. Below 0.6V on oscillator, logic, current amplifi er and 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 fl yback error amplifi er and a temperature compensation decreases in frequency. With the decrease of the internal circuit. In addition, the logic system contains additional oscillator, the part starts to operate in DCM. The output logic for boundary mode operation, and the sampling current is able to decrease while still allowing a minimum error amplifi er. switch off-time for the error amp sampling circuitry. The typical minimum internal oscillator frequency with VC equal to 0V is 40kHz. 3575f 7