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LT3573E

Isolated Flyback Converter without an Opto-Coupler

Linear Technology 

LT3573

OPERATION

The LT3573 is a current mode switching regulator IC

designed specifically for the isolated flyback topology.

The special problem normally encountered in such cir-

cuits 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 LT3573 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. Since this

IC operates in boundary control mode, the output voltage

is calculated from the switch pin when the secondary cur-

rent 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: internal bias regulator,

oscillator, logic, current amplifier and comparator, driver,

and output switch. The novel sections include a special

flyback error amplifier and a temperature compensation

circuit. In addition, the logic system contains additional

logic for boundary mode operation, and the sampling

error amplifier.

The LT3573 features a boundary mode control method,

where the part operates at the boundary between continu-

ous conduction mode and discontinuous conduction mode.

The VC pin controls the current level just as it does in normal

current mode operation, but instead of turning the switch

on at the start of the oscillator period, the part detects

when the secondary side winding current is zero.

Boundary Mode Operation

Boundary mode is a variable frequency, current-mode

switching scheme. The switch turns on and the inductor

current increases until a VC pin controlled current limit. The

voltage on the SW pin rises to the output voltage divided

by the secondary-to-primary transformer turns ratio plus

the input voltage. When the secondary current through

the diode falls to zero, the SW pin voltage falls below VIN.

A discontinuous conduction mode (DCM) comparator

detects this event and turns the switch 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

conduction mode and no subharmonic oscillation.

At low output currents the LT3573 delays turning on the

switch, and thus operates in discontinuous mode. Unlike

a traditional flyback converter, the switch 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 the internal oscillator frequency

decreases in frequency. With the decrease of the internal

oscillator, the part starts to operate in DCM. The output

current is able to decrease while still allowing a minimum

switch off-time for the error amp sampling circuitry. The

typical minimum internal oscillator frequency with VC

equal to 0V is 40kHz.

For more information www.linear.com/LT3573

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