LT3748E 查看數據表（PDF） - Linear Technology

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LT3748E

100V Isolated Flyback Controller

Linear Technology 

LT3748

Operation

The LT3748 is a current mode switching regulator con-

troller designed specifically for the isolated flyback topol-

ogy. The special problem normally encountered in such

circuits is that information relating to the output voltage

on the isolated secondary side of the transformer must

be communicated to the primary side in order to maintain

regulation. Historically, this has been done with opto-

isolators or extra transformer windings. Opto-isolator

circuits waste output power and the extra components

increase the cost and physical size of the power supply.

Opto-isolators can also exhibit trouble due to limited

dynamic response, nonlinearity, unit-to-unit variation

and aging over life. Circuits employing extra transformer

windings also exhibit deficiencies. Using an extra wind-

ing adds to the transformer’s physical size and cost, and

dynamic response is often mediocre.

The LT3748 derives its information about the isolated

output voltage by examining the primary-side flyback

pulse waveform. In this manner, no opto-isolator nor

extra transformer winding is required for regulation. The

output voltage is easily programmed with two resistors.

The LT3748 features a boundary mode control method,

(also called critical conduction mode) where the part

operates at the boundary between continuous conduc-

tion mode and discontinuous conduction mode. Due to

the boundary control mode operation, the output voltage

can be calculated from the transformer primary voltage

when the secondary current is almost zero. This method

improves load regulation without external resistors and

capacitors.

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.

Boundary Mode Operation

Boundary mode is a variable frequency, current mode

switching scheme. The external N-channel MOSFET turns

on and the inductor current increases until it reaches the VC

pin-controlled current limit. After the external MOSFET is

turned off, the voltage on the drain of the MOSFET rises to

the output voltage multiplied by the primary-to-secondary

transformer turns ratio plus the input voltage. When the

secondary current through the output diode falls to zero,

the voltage on the drain of the MOSFET falls below VIN. A

boundary mode detection comparator detects this event

and turns the external MOSFET back on.

Boundary mode returns the secondary current to zero every

cycle, so the parasitic resistive voltage drops do not cause

load regulation errors. Boundary mode also allows the use

of a smaller transformer compared to continuous conduc-

tion mode and does not exhibit subharmonic oscillation.

At low output currents the LT3748 delays turning on the

external MOSFET and thus operates in discontinuous mode.

Unlike traditional flyback converters, the external MOSFET

has to turn on to update the output voltage information.

Below 0.6V on the VC pin, the current comparator level

decreases to its minimum value and a variable delay timer

waits to reset before turning on the external MOSFET. With

the addition of delay before turning the MOSFET back

on, the part starts to operate in discontinuous mode. The

average output current is able to decrease while still allow-

ing a minimum off-time for the error amplifier sampling

circuitry. The typical maximum discontinuous off-time

with VC equal to 0V is 24µs.

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For more information www.linear.com/LT3748

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