LT3799IMSE-1(RevA) 查看數據表（PDF） - Linear Technology

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LT3799IMSE-1
(Rev.:RevA)

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799-1

Operation

When the VIN_SENSE voltage is connected to a resistor di-

vider of the supply voltage, the current limit is proportional

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 LT3799-1 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.

Start-Up

The LT3799-1 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 at-

tached 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)

•

42

•

N

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.

For more information www.linear.com/LT3799-1

37991fa

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