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LT3799EMSE

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799

OPERATION

MOSFET off when the TRIAC is off, this power device is

kept on and sinks the current to properly load the TRIAC.

When the TRIAC turns on, the VIN_SENSE pin detects this

and enables the loop, but the current comparator is always

enabled and turns the switch off if it is tripped.

Start-Up

The LT3799 uses a hysteretic start-up to operate from

high offline voltages. A resistor connected to the supply

voltage protects the part from high voltages. This resis-

tor is connected to the VIN pin on the part and also to a

capacitor. When the resistor charges the part up to 23V and

INTVCC is in regulation at 10V, the part begins to charge the

CT pin to 340mV and then starts to switch. The resistor

does not provide power for the part in steady state, but

relies on the capacitor to start-up the part, then the third

winding begins to provide power to the VIN pin along with

the resistor. An internal voltage clamp is attached to the

VIN pin to prevent the resistor current from allowing VIN

to go above the absolute maximum voltage of the pin.

The internal clamp is set at 25V and is capable of 28mA

(typical) of current at room temperature. But, ideally, the

resistor connected between the input supply and the VIN

pin should be chosen so that less than 10mA is being

shunted by this internal clamp.

CT Pin and Faults

The CT pin is a timing pin for the fault circuitry. When the

input voltages are at the correct levels, the CT pin sources

10µA of current. When the CT pin reaches 340mV, the part

begins to switch. The output voltage information from the

FB pin is sampled but ignored until the CT pin reaches

1.25V. When this occurs, if the FB pin is above 1.25V, the

fault flag pulls low. The FAULT pin is meant to be used

with a large pull-up resistor to the INTVCC pin or another

supply. The CT pin begins to sink 200nA of current. When

the CT pin goes below 240mV, the part will re-enable itself,

begin to switch, and start to source 10µA of current to the

CT pin but not remove the fault condition. When the CT

pin reaches 1.25V and FB is below 1.25V, the FAULT pin

will no longer pull low and switching will continue. If not

below 1.25V, the process repeats itself.

Programming Output Current

The maximum output current depends on the supply

voltage and the output voltage in a flyback topology.

With the VIN_SENSE pin connected to 1V and a DC supply

voltage, the maximum output current is determined at

the minimum supply voltage, and the maximum output

voltage using the following equation:

IO U T(M A X )

=

2

• (1− D) •

N

42 • RSENSE

where

D = VOUT • N

VOUT • N + VIN

The maximum control voltage to achieve this maximum

output current is 2V • (1-D).

It is suggested to operate at 95% of these values to give

margin for the part’s tolerances.

When designing for power factor correction, the output

current waveform is going to have a half sine wave squared

shape and will no longer be able to provide the above

currents. By taking the integral of a sine wave squared

over half a cycle, the average output current is found to

be half the value of the peak output current. In this case,

the recommended maximum average output current is

as follows:

IO U T(M A X )

= (1− D) •

N

42 • RSENSE

•

47.5%

where

D = VOUT • N

VOUT • N + VIN

The maximum control voltage to achieve this maximum

output current is (1-D) • 47.5%.

For control voltages below the maximum, the output cur-

rent is equal to the following equation:

IO U T

=

CTRL

•

42

N

• RSENSE

3799p

11

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