LT3799EMSE-1-PBF Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

Список матч

LT3799EMSE-1-PBF

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799-1

OPERATION

The LT3799-1 is a current mode switching controller IC

designed specifically for generating an average current

output in an isolated flyback topology. The special problem

normally encountered in such circuits is that information

relating to the output voltage and current on the isolated

secondary side of the transformer must be communicated

to the primary side in order to maintain regulation. Histori-

cally, this has been done with an opto-isolator. The LT3799-1

uses a novel method of using the external MOSFETs peak

current information from the sense resistor to calculate

the output current of a flyback converter without the need

of an opto-coupler. In addition, it also detects open LED

conditions by examining the third winding voltage when

the main power switch is off.

Power factor has become an important specification for

lighting. A power factor of one is achieved if the current

drawn is proportional to the input voltage. The LT3799-1

modulates the peak current limit with a scaled version of

the input voltage. This technique provides power factors

of 0.97 or greater.

The Block Diagram shows an overall view of the system.

The external components are in a flyback topology con-

figuration. The third winding senses the output voltage

and also supplies power to the part in steady-state opera-

tion. The VIN pin supplies power to an internal LDO that

generates 10V at the INTVCC pin. The novel control circuitry

consists of an error amplifier, a multiplier, a transmission

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 LT3799-1 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

37991f

9

Share Link: