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LT3799EMSE

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799

OPERATION

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 transformer.

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 winding 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 current and zero dur-

SECONDARY

DIODE CURRENT

IPK(sec)

SWITCH

WAVEFORM

TFLYBACK

TPERIOD

3799 F01

Figure 1. Secondary Diode Current and Switch Waveforms

10

ing the rest of the cycle. The equation for expressing the

output current is:

IOUT = 0.5 • IPK • N • D

where D is equal to the percentage of the cycle represented

by the flyback time.

The LT3799 has access to both the primary winding cur-

rent, the input to the current comparator, and when the

flyback time starts and ends. Now the output current can

be calculated by averaging a PWM waveform with the

height of the current limit and the duty cycle of the flyback

time over the entire cycle. In the feedback loop previously

described, the input to the integrator is such a waveform.

The integrator adjusts the peak current until the calculated

output current equals the control voltage. If the calculated

output current is low compared to the control pin, the error

amplifier increases the voltage on the COMP+ node, thus

increasing the current comparator input.

When the VIN_SENSE voltage is connected to a resistor

divider of the supply voltage, the current limit is propor-

tional to the supply voltage if COMP+ is held constant.

The output of the error amplifier is multiplied with the

VIN_SENSE pin voltage. If the LT3799 is configured with a

fast control loop, slower changes from the VIN_SENSE pin

will not interfere with the current limit or the output current.

The COMP+ pin will adjust to the changes of the VIN_SENSE.

The only way for the multiplier to function properly is to

set the control loop to be an order of magnitude slower

than the fundamental frequency of the VIN_SENSE signal. In

the offline case, the fundamental frequency of the supply

voltage is 120Hz, so the control loop unity gain frequency

needs to be set less than approximately 120Hz. Without a

large amount of energy storage on the secondary side, the

output current is affected by the supply voltage changes,

but the DC component of the output current is accurate.

TRIAC Dimming Features

The LT3799 incorporates some special features that aid in

the design of an offline LED current source when used with

a TRIAC dimmer. TRIAC dimmers are not ideal switches

when turned off and allow milliamps of current to flow

through them. This is an issue if used with a low quiescent

part such as the LT3799. Instead of turning the main power

3799p

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