Datasheet LT3573 (Analog Devices) - 7

LT3573
OPERATION
The LT3573 is a current mode switching regulator IC The LT3573 features a boundary mode control method, designed specifically for the isolated flyback topology. where the part operates at the boundary between continu- The special problem normally encountered in such cir- ous conduction mode and discontinuous conduction mode. cuits is that information relating to the output voltage on The VC pin controls the current level just as it does in normal the isolated secondary side of the transformer must be current mode operation, but instead of turning the switch communicated to the primary side in order to maintain on at the start of the oscillator period, the part detects regulation. Historically, this has been done with opto- when the secondary side winding current is zero. isolators or extra transformer windings. Opto-isolator circuits waste output power and the extra components
Boundary Mode Operation
increase the cost and physical size of the power supply. Boundary mode is a variable frequency, current-mode Opto-isolators can also exhibit trouble due to limited switching scheme. The switch turns on and the inductor dynamic response, nonlinearity, unit-to-unit variation current increases until a VC pin control ed current limit. The and aging over life. Circuits employing extra transformer voltage on the SW pin rises to the output voltage divided windings also exhibit deficiencies. Using an extra wind- by the secondary-to-primary transformer turns ratio plus ing adds to the transformer’s physical size and cost, and the input voltage. When the secondary current through dynamic response is often mediocre. the diode falls to zero, the SW pin voltage falls below VIN. A discontinuous conduction mode (DCM) comparator The LT3573 derives its information about the isolated detects this event and turns the switch back on. output voltage by examining the primary-side flyback pulse waveform. In this manner, no opto-isolator nor extra Boundary mode returns the secondary current to zero transformer winding is required for regulation. The output every cycle, so the parasitic resistive voltage drops do not voltage is easily programmed with two resistors. Since this cause load regulation errors. Boundary mode also allows IC operates in boundary control mode, the output voltage the use of a smaller transformer compared to continuous is calculated from the switch pin when the secondary cur- conduction mode and no subharmonic oscillation. rent is almost zero. This method improves load regulation At low output currents the LT3573 delays turning on the without external resistors and capacitors. switch, and thus operates in discontinuous mode. Unlike The Block Diagram shows an overall view of the system. a traditional flyback converter, the switch has to turn on Many of the blocks are similar to those found in traditional to update the output voltage information. Below 0.6V on switching regulators including: internal bias regulator, the VC pin, the current comparator level decreases to oscillator, logic, current amplifier and comparator, driver, its minimum value, and the internal oscillator frequency and output switch. The novel sections include a special decreases in frequency. With the decrease of the internal flyback error amplifier and a temperature compensation oscillator, the part starts to operate in DCM. The output circuit. In addition, the logic system contains additional current is able to decrease while still allowing a minimum logic for boundary mode operation, and the sampling switch off-time for the error amp sampling circuitry. The error amplifier. typical minimum internal oscillator frequency with VC equal to 0V is 40kHz. 3573fd For more information www.linear.com/LT3573 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