LT3798
16
3798fa
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 V
IN
UVLO to shutdown
switching. The part starts switching again when V
IN
has
reached its turn-on voltage.
OPERATION
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 V
IN
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 V
IN_SENSE
to INTV
CC
.
V
3RD WINDING
20V/DIV
V
OUT
10V/DIV
I
OUT
1A/DIV
3798 F05
1ms/DIV
Figure 5. Switching Waveforms When Output
Open-Circuits or at Very Light Load Conditions
Figure 6. Switching Waveforms When Output Short-Circuits
V
IN
20V/DIV
V
3RD WINDING
50V/DIV
I
PRI
1A/DIV
3798 F06
100ms/DIV
