3575FE 查看數據表（PDF） - Linear Technology

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3575FE

Isolated Flyback Converter without an Opto-Coupler

Linear Technology 

LT3575

OPERATION

The LT3575 is a current mode switching regulator IC

designed specifically for the isolated flyback topology. 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 optoisolators or extra transformer

windings. 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 extra

transformer windings also exhibit deficiencies. Using an

extra winding adds to the transformer’s physical size and

cost, and dynamic response is often mediocre.

The LT3575 derives its information about the isolated

output voltage by examining the primary side flyback

pulse waveform. In this manner, no optoisolator 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 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: 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 LT3575 features a boundary mode control method,

where the part operates at the boundary between continuous

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 LT3575 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.

3575f

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