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LT3799EMSE-1-PBF

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799-1

OPERATION

Sense Resistor Selection

The resistor, RSENSE, between the source of the external

N-channel MOSFET and GND should be selected to provide

an adequate switch current to drive the application without

exceeding the current limit threshold .

For applications without power factor correction, select a

resistor according to:

RSENSE

=

2(1− D)N

IOUT • 42

•

95%

where

D = VOUT • N

VOUT • N + VIN

For applications with power factor correction, select a

resistor according to:

RSENSE

=

2(1− D)N

IOUT • 42

•

47.5%

where

D = VOUT • N

VOUT • N + VIN

Minimum Current Limit

The LT3799-1 features a minimum current limit of ap-

proximately 7% of the peak current limit. This is necessary

when operating in critical conduction mode since low

current limits would increase the operating frequency to a

very high frequency. The output voltage sensing circuitry

needs a minimum amount of flyback waveform time to

sense the output voltage on the third winding. The time

needed is 350ns. The minimum current limit allows the

use of smaller transformers since the magnetizing primary

inductance does not need to be as high to allow proper

time to sample the output voltage information.

Errors Affecting Current Output Regulation

There are a few factors affecting the regulation of current in

a manufacturing environment along with some systematic

issues. The main manufacturing issues are the winding

turns ratio and the LT3799-1 control loop accuracy. The

winding turns ratio is well controlled by the transformer

manufacturer’s winding equipment, but most transformers

do not require a tight tolerance on the winding ratio. We

have worked with transformer manufacturers to specify

±1% error for the turns ratio. Just like any other LED driver,

the part is tested and trimmed to eliminate offsets in the

control loop and an error of ±3% is specified at 80% of

the maximum output current. The error grows larger as

the LED current is decreased from the maximum output

current. At half the maximum output current, the error

doubles to ±6%.

There are a number of systematic offsets that may be elimi-

nated by adjusting the control voltage from the ideal voltage.

It is difficult to measure the flyback time with complete

accuracy. If this time is not accurate, the control voltage

needs to be adjusted from the ideal value to eliminate the

offset but this error still causes line regulation errors. If

the supply voltage is lowered, the time error becomes a

smaller portion of the switching cycle period so the offset

becomes smaller and vice versa. This error may be com-

pensated for at the primary supply voltage, but this does

not solve the problem completely for other supply voltages.

Another systematic error is that the current comparator

cannot instantaneously turn off the main power device.

This delay time leads to primary current overshoot. This

overshoot is less of a problem when the output current is

close to its maximum, since the overshoot is only related

to the slope of the primary current and not the current

level. The overshoot is proportional to the supply voltage,

so again this affects the line regulation.

37991f

13

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