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LT3511

3511fa

operaTion

The LT3511 is a current mode switching regulator IC de-

signed specifically for the isolated flyback topology. The

key problem in isolated topologies is how to communicate

information regarding the output voltage from the isolated

secondary side of the transformer to the primary side.

Historically, optoisolators or extra transformer windings

communicate this information across the transformer.

Optoisolator circuits waste output power, and the extra

components increase the cost and physical size of the

power supply. Optoisolators can also exhibit trouble due

to limited dynamic response, nonlinearity, unit-to-unit

variation and aging over life. Circuits employing an extra

transformer winding also exhibit deficiencies. Using an

extra winding adds to the transformer’s physical size and

cost, and dynamic response is often mediocre.

In the LT3511, the primary-side flyback pulse provides

information about the isolated output voltage. In this man-

ner, neither optoisolator nor extra transformer winding is

required for regulation. Two resistors program the output

voltage. Since this IC operates in boundary mode, the part

calculates output voltage from the switch pin when the

secondary current is almost zero.

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, os-

cillator, logic, current amplifier, current 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.

The LT3511 features boundary mode control, where the

part operates at the boundary between continuous con-

duction 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 turns on the

switch 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.

After the switch turns off, 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.

Boundarymodereturnsthesecondarycurrenttozeroevery

cycle,soparasiticresistivevoltagedropsdonotcauseload

regulation errors. Boundary mode also allows the use of a

smaller transformer compared to continuous conduction

mode and does not exhibit subharmonic oscillation.

At low output currents, the LT3511 delays turning on the

switch, and thus operates in discontinuous mode. Unlike

traditional flyback converters, 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. 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 flyback error amplifier. The typical minimum

internal oscillator frequency with VC equal to 0V is 40kHz.

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