Datasheet

LT3751
21
3751fc
Figure 11. Effects of RC Snubber
Figure 10. RC Snubber Circuit
applicaTions inForMaTion
unnecessarily high V
(BR)DSS
which equates to a larger
R
DS(ON)
. Secondly, the V
DRAIN
node will ring—possibly
below ground—causing false tripping of the DCM com-
parator or damage to the NMOS switch (see Figure 11).
Both issues can be remedied using a snubber. If leakage
inductance causes issues, it is recommended to use a RC
snubber in parallel with the primary winding, as shown
in Figure 10. Size C
SNUB
and R
SNUB
based on the desired
leakage spike voltage, known leakage inductance, and an
RC time constant less than 1µs. Otherwise, the leakage
voltage spike can cause false tripping of the V
OUT
com-
parator and stop charging prematurely.
Figure 11 shows the effect of the RC snubber resulting in
a lower voltage spike and faster settling time.
3751 F11
L
PRI
R
SNUB
C
SNUB
L
LEAK
•
•
C
VDRAIN
0V
0V
3751 F12
V
DRAIN
(WITH
SNUBBER)
V
DRAIN
(WITHOUT
SNUBBER)
I
PRI
NMOS DIODE
CONDUCTS
LOW NOISE REGULATION
The LT3751 has the option to provide a low noise regu-
lated output voltage when using a resistive voltage divider
from the output node to the FB pin. Refer to the Selecting
Component Parameters section to design the transformer,
NMOS power switch, output diode, and sense resistor.
Use the following equations to select the feedback resis-
tor values based on the power dissipation and desired
output voltage:
R
FBH
=
V
OUT
− 1.22
( )
2
P
D
; Top Feedback Resistor
R
FBL
=
1.22
V
OUT
− 1.22






• R
FBH
; Bottom Feedback Resistor
R
FBH
, depending on output voltage and type used, may
require several smaller values placed in series. This will
reduce the risk of arcing and damage to the feedback resis-
tors. Consult the manufacturer’s rated voltage specification
for safe operation of the feedback resistors.
The LT3751 has a minimum periodic refresh frequency
limit of 23kHz. This drastically reduces switching frequency
components in the audio spectrum. The LT3751 can oper-
ate with no-load, but the regulation scheme switches to
no-load operation and audible noise and output voltage
ripple increase. This can be avoided by operating with a
minimum load current.
Minimum Load Current
Periodic refresh circuitry requires an average minimum
load current to avoid entering no-load operation. Usually,
the feedback resistors should be adequate to provide this
minimum load current.
I
LOAD(MIN)
≥
L
PRI
• I
2
PK
• 23kHz
100 • V
OUT
I
PK
is the peak primary current at maximum power delivery.
The LT3751 will enter no-load operation if the minimum
load current is not met. No-load operation will prevent the
application from entering a runaway condition; however,
the output voltage will increase 10% over the nominal
regulated voltage.