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LT3798IMSE-PBF

Isolated No Opto-Coupler Flyback Controller with Active PFC

Linear Technology 

LT3798

OPERATION

Protection from Shorted Output Conditions

During a shorted output condition as shown in Figure 6, the

LT3798 operates at the minimum operating frequency. In

normal operation, the third winding provides power to the

IC, but the third winding voltage is zero during a shorted

condition. This causes the part’s VIN UVLO to shutdown

switching. The part starts switching again when VIN has

reached its turn-on voltage.

V3RD WINDING

20V/DIV

VOUT

10V/DIV

Usage with DC Input Voltage

The LT3798 is flexible enough to operate well from low

voltage to very high voltage DC input voltage applications.

When the supply voltage is less than 40V, the startup re-

sistor is not needed and the part's VIN can be connected

directly to the supply voltage. The startup sequence for

voltages higher than 40V is the same as what is described

for high voltage offline supply voltages.

The loop compensation component values can be chosen

to provide faster loop response since the LT3798 does

not have to provide PFC for the slow 50Hz/60Hz AC input

voltage. For DC input applications, connect a 25k resistor

from VIN_SENSE to INTVCC.

IOUT

1A/DIV

1ms/DIV

3798 F05

Figure 5. Switching Waveforms When Output

Open-Circuits or at Very Light Load Conditions

VIN

20V/DIV

V3RD WINDING

50V/DIV

IPRI

1A/DIV

100ms/DIV

3798 F06

Figure 6. Switching Waveforms When Output Short-Circuits

3798fa

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