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LT3798EMSE

Isolated No Opto-Coupler Flyback Controller with Active PFC

Linear Technology 

LT3798

OPERATION

fundamental frequency of the supply voltage is 120Hz so

the control loop unity gain frequency needs to be set less

than approximately 12Hz. Without a large amount of energy

storage on the secondary side, the output current will be

affected by the supply voltage changes, but the DC com-

ponent of the output current will be accurate. For DC input

or non-PFC AC input applications, connect a 25k resistor

from VIN_SENSE to INTVCC instead of the AC line voltage.

Startup

The LT3798 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 resistor

is connected to the VIN pin on the part and bypassed with

a capacitor. When the resistor charges the VIN pin to a

turn-on voltage set with the EN/UVLO resistor divider and

the INTVCC pin is at its regulation point, the part begins

to switch. The resistor cannot 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 40V and is

capable of 8mA(typical) of current at room temperature.

Setting the VIN Turn-On and Turn-Off Voltages

A large voltage difference between the VIN turn-on voltage

and the VIN turn-off voltage is preferred to allow time for the

third winding to power the part. The EN/UVLO sets these

two voltages. The pin has a 10µA current sink when the

pins voltage is below 1.25V and 0µA when above 1.25V.

The VIN pin connects to a resistor divider as shown in

Figure 2. The UVLO threshold for VIN rising is:

VI N(U V L O,R I S I N G)

=

1.25V

• (R1+

R2

R2)

+

10µA

• R1

The UVLO Threshold for VIN Falling is :

VI N(U V L O,F A L L I N G)

=

1.25V

• (R1+

R2

R2)

EN/UVLO

VIN

R1

LT3798

R2

GND

3798 F02

Figure 2. Undervoltage Lockout (UVLO)

Programming Output Voltage

The output voltage is set using a resistor divider from

the third winding to the FB pin. From the Block Diagram,

the resistors R4 and R5 form a resistor divider from the

third winding. The FB also has an internal current source

that compensates for the diode drop. This current source

causes an offset in the output voltage that needs to be ac-

counted for when setting the output voltage. The output

voltage equation is:

VOUT = VBG (R4+R5)/(NST • R5)–(VF + (R4 • ITC)/NST)

where VBG is the internal reference voltage, NST is the

winding ratio between the secondary winding and the third

winding, VF is the forward drop of the output rectifying

diode, and ITC is the internal current source for the FB pin.

The temperature coefficient of the diode's forward drop

needs to be the opposite of the term, (R4 • ITC)/NST. By

taking the partial derivative with respect to temperature,

the value of R4 is found to be the following:

R4 = NST(1/(δITC/δT)(δVF/δT))

δITC/δT = 12.4nA/°C

ITC = 4.25µA

where δITC/δT is the partial derivative of the ITC current

source, and δVF/δT is the partial derivative of the forward

drop of the output rectifying diode.

With R4 set with the above equation, the resistor value

for R5 is found using the following:

R5 = (VBG • R4)/(NST(VOUT+VF)+R4 • ITC-VBG)

3798fa

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