LT8301MPS5-TRPBF 查看數據表（PDF） - Linear Technology

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LT8301MPS5-TRPBF

42VIN Micropower No-Opto Isolated Flyback Converter with 65V/1.2A Switch

Linear Technology 

LT8301

OPERATION

The LT8301 is a simple to use micropower isolated flyback

converter housed in a 5-lead TSOT-23 package. The output

voltage is programmed with a single external resistor. By

integrating the loop compensation and soft-start inside, the

part further reduces the number of external components.

As shown in the Block Diagram, many of the blocks are

similar to those found in traditional switching regulators

including reference, regulators, oscillator, logic, current

amplifier, current comparator, driver, and power switch.

The novel sections include a flyback pulse sense circuit,

a sample-and-hold error amplifier, and a boundary mode

detector, as well as the additional logic for boundary

conduction mode, discontinuous conduction mode, and

low ripple Burst Mode operation.

Boundary Conduction Mode Operation

The LT8301 features boundary conduction mode operation

at heavy load, where the chip turns on the primary power

switch when the secondary current is zero. Boundary

conduction mode is a variable frequency, variable peak-

current switching scheme. The power switch turns on

and the transformer primary current increases until an

internally controlled peak current limit. After the power

switch turns off, the voltage on the SW pin 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 SW pin voltage collapses and rings around VIN. A

boundary mode detector senses this event and turns the

power switch back on.

Boundary conduction mode returns the secondary current

to zero every cycle, so parasitic resistive voltage drops

do not cause load regulation errors. Boundary conduc-

tion mode also allows the use of smaller transformers

compared to continuous conduction mode and does not

exhibit sub-harmonic oscillation.

Discontinuous Conduction Mode Operation

As the load gets lighter, boundary conduction mode in-

creases the switching frequency and decreases the switch

peak current at the same ratio. Running at a higher switching

frequency up to several MHz increases switching and gate

charge losses. To avoid this scenario, the LT8301 has an

additional internal oscillator, which clamps the maximum

switching frequency to be less than 430kHz (typ). Once

the switching frequency hits the internal frequency clamp,

the part starts to delay the switch turn-on and operates in

discontinuous conduction mode.

Low Ripple Burst Mode Operation

Unlike traditional flyback converters, the LT8301 has to

turn on and off at least for a minimum amount of time

and with a minimum frequency to allow accurate sampling

of the output voltage. The inherent minimum switch cur-

rent limit and minimum switch-off time are necessary to

guarantee the correct operation of specific applications.

As the load gets very light, the LT8301 starts to fold back

the switching frequency while keeping the minimum switch

current limit. So the load current is able to decrease while

still allowing minimum switch-off time for the sample-

and-hold error amplifier. Meanwhile, the part switches

between sleep mode and active mode, thereby reducing the

effective quiescent current to improve light load efficiency.

In this condition, the LT8301 operates in low ripple Burst

Mode. The 10kHz (typ) minimum switching frequency

determines how often the output voltage is sampled and

also the minimum load requirement.

8301f

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For more information www.linear.com/LT8301

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