LT3799I(RevA) Просмотр технического описания (PDF) - Linear Technology

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LT3799I
(Rev.:RevA)

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799

Operation

gate, a current comparator, a low output current oscillator

and a master latch, which will be explained in the follow-

ing sections. The part also features a sample-and-hold

to detect open LED conditions, along with a FAULT pin. A

comparator is used to detect discontinuous conduction

mode (DCM) with a cap connected to the third winding.

The part features a 1.9A gate driver.

The LT3799 employs a micropower hysteretic start-up

feature to allow the part to work at any combination of

input and output voltages. In the Block Diagram, R3 is used

to stand off the high voltage supply voltage. The internal

LDO starts to supply current to the INTVCC when VIN is

above 23V. The VIN and INTVCC capacitors are charged by

the current from R3. When VIN exceeds 23V and INTVCC is

in regulation at 10V, the part will began to charge the CT

pin with 10µA. Once the CT pin reaches 340mV, switching

begins. The VIN pin has 10.7V of hysteresis to allow for

plenty of flexibility with the input and output capacitor

values. The third winding provides power to VIN when its

voltage is higher than the VIN voltage. A voltage shunt is

provided for fault protection and can sink up to 15mA of

current when VIN is over 25V.

During a typical cycle, the gate driver turns the external

MOSFET on and a current flows through the primary

winding. This current increases at a rate proportional

to the input voltage and inversely proportional to the

magnetizing inductance of the transformer. The control

loop determines the maximum current and the current

comparator turns the switch off when the current level

is reached. When the switch turns off, the energy in the

core of the transformer flows out the secondary winding

through the output diode, D1. This current decreases at a

rate proportional to the output voltage. When the current

decreases to zero, the output diode turns off and voltage

across the secondary winding starts to oscillate from the

parasitic capacitance and the magnetizing inductance of

the transformer. Since all windings have the same voltage

across them, the third winding rings too. The capacitor

connected to the DCM pin, C1, trips the comparator, A2,

which serves as a dv/dt detector, when the ringing occurs.

This timing information is used to calculate the output

current (description to follow). The dv/dt detector waits

for the ringing waveform to reach its minimum value and

then the switch turns back on. This switching behavior is

similar to zero volt switching and minimizes the amount of

energy lost when the switch is turned back on, improving

efficiency as much as 5%. Since this part operates on the

edge of continuous conduction mode and discontinuous

conduction mode, this operating mode is called critical

conduction mode (or boundary conduction mode).

Primary-Side Current Control Loop

The CTRL1/CTRL2/CTRL3 pins control the output current

of the flyback controller. To simplify the loop, assume

the VIN_SENSE pin is held at a constant voltage above

1V, eliminating the multiplier from the control loop. The

error amplifier, A5, is configured as an integrator with

the external capacitor, C6. The COMP+ node voltage is

converted to a current into the multiplier with the V/I

converter, A6. Since A7’s output is constant, the output

of the multiplier is proportional to A6 and can be ignored.

The output of the multiplier controls the peak current with

its connection to the current comparator, A1. The output

of the multiplier is also connected to the transmission

gate, SW1. The transmission gate, SW1, turns on when

the secondary current flows to the output capacitor. This

is called the flyback period (when the output diode D1 is

on). The current through the 1M resistor gets integrated

by A5. The lowest CTRL input is equal to the negative input

of A5 in steady state.

A current output regulator normally uses a sense resistor

in series with the output current and uses a feedback loop

to control the peak current of the switching converter. In

this isolated case the output current information is not

available, so instead the LT3799 calculates it using the

information available on the primary side of the trans-

former. The output current may be calculated by taking the

average of the output diode current. As shown in Figure

1, the diode current is a triangle waveform with a base of

the flyback time and a height of the peak secondary wind-

ing current. In a flyback topology, the secondary winding

current is N times the primary winding current, where

N is the primary to secondary winding ratio. Instead of

taking the area of the triangle, think of it as a pulse width

modulation (PWM) waveform. During the flyback time,

the average current is half the peak secondary winding

3799fa

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