Datasheet LT3748 (Analog Devices) - 8

LT3748
operation
The LT3748 is a current mode switching regulator con-
Boundary Mode Operation
troller designed specifically for the isolated flyback topol- Boundary mode is a variable frequency, current mode ogy. The special problem normally encountered in such switching scheme. The external N-channel MOSFET turns circuits is that information relating to the output voltage on and the inductor current increases until it reaches the V on the isolated secondary side of the transformer must C pin-controlled current limit. After the external MOSFET is be communicated to the primary side in order to maintain turned off, the voltage on the drain of the MOSFET rises to regulation. Historically, this has been done with opto- the output voltage multiplied by the primary-to-secondary isolators or extra transformer windings. Opto-isolator transformer turns ratio plus the input voltage. When the circuits waste output power and the extra components secondary current through the output diode falls to zero, increase the cost and physical size of the power supply. the voltage on the drain of the MOSFET falls below VIN. A Opto-isolators can also exhibit trouble due to limited boundary mode detection comparator detects this event dynamic response, nonlinearity, unit-to-unit variation and turns the external MOSFET back on. and aging over life. Circuits employing extra transformer windings also exhibit deficiencies. Using an extra wind- Boundary mode returns the secondary current to zero every ing adds to the transformer’s physical size and cost, and cycle, so the parasitic resistive voltage drops do not cause load regulation errors. Boundary mode also allows the use dynamic response is often mediocre. of a smaller transformer compared to continuous conduc- The LT3748 derives its information about the isolated tion mode and does not exhibit subharmonic oscillation. output voltage by examining the primary-side flyback At low output currents the LT3748 delays turning on the pulse waveform. In this manner, no opto-isolator nor external MOSFET and thus operates in discontinuous mode. extra transformer winding is required for regulation. The Unlike traditional flyback converters, the external MOSFET output voltage is easily programmed with two resistors. has to turn on to update the output voltage information. The LT3748 features a boundary mode control method, Below 0.6V on the V (also called critical conduction mode) where the part C pin, the current comparator level decreases to its minimum value and a variable delay timer operates at the boundary between continuous conduc- waits to reset before turning on the external MOSFET. With tion mode and discontinuous conduction mode. Due to the addition of delay before turning the MOSFET back the boundary control mode operation, the output voltage on, the part starts to operate in discontinuous mode. The can be calculated from the transformer primary voltage average output current is able to decrease while still allow- when the secondary current is almost zero. This method ing a minimum off-time for the error amplifier sampling improves load regulation without external resistors and circuitry. The typical maximum discontinuous off-time capacitors. with VC equal to 0V is 24µs. The Block Diagram shows an overall view of the system. Many of the blocks are similar to those found in traditional switching regulators, including current comparators, inter- nal reference and regulators, logic, timers and an N-channel MOSFET gate driver. The novel sections include a special sampling error amplifier and a temperature compensation circuit. 3748fb 8 For more information www.linear.com/LT3748 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 Package Description Typical Application Related Parts