Datasheet

LTC3765
14
3765fb
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APPLICATIONS INFORMATION
Choose the overcurrent trip current I
TRIP
to be less than
the maximum pulsed drain current rating of the primary
NMOS but greater than the sum of the peak inductor current
at full load and the current required to charge the output
capacitor at start-up, reflected through the transformer.
The sense resistor value, R
SENSE
, can be calculated from
the 150mV trip threshold and the primary-side trip current
from the following equation:
R
SENSE
=
150mV
I
TRIP
The R
SENSE
resistor should be verified to have sufficient
margin over the maximum operating current of the con-
verter, which typically occurs at start-up into a full load.
In
a self-starting application at full load, the linearly increas-
ing duty
cycle determined by the soft-start capacitor (C
SS
)
ramps the output voltage with a fixed rate independent
of the output capacitor value. Larger output capacitance
requires a proportionally larger charging current to main
-
tain the output voltage ramp rate. Since additional output
capacitance
is generally distributed through a system and
may not be exactly known, the sense resistor and soft-
start capacitor should be chosen with sufficient margin
to ensure that the overcurrent comparator does not trip
on start-up. An upper bound for the available current to
charge the output capacitor can be calculated as shown
in the following equation:
I
CHG
<
150mV
R
SENSE
•N
S
/N
P
– 1.4
I
LOAD(MAX)
where N
S
/N
P
is the turns ratio of the transformer and the
factor of 1.4 accounts for the typical 40% ripple current
in the inductor. To ensure that the overcurrent comparator
does not trip on start-up, the soft-start capacitor should
be chosen so that only a fraction of the charging cur
-
rent calculated
above is available to charge the output
capacitor. Using 10% of the maximum charging current
generally allows for sufficient margin. This establishes a
lower bound on the soft-start capacitor value, which can
be computed from:
C
SS
> 600 •10
–9
• V
IN(MAX)
•N
S
/N
P
C
OUT
0.1•I
CHG






where C
OUT
is the output capacitor value and V
IN(MAX)
is
the maximum input voltage. The soft-start capacitor value
should be in the range of 10nF to 1µF. Do not use a value
less than 10nF. The soft-start capacitor also determines the
relative timing of the output voltage rise and the LTC3766
bias supply rise. The value chosen should be verified with
the equations in the following Self-Starting Start-Up sec
-
tion to ensure that the bias supply rises before the output
voltage is close to the regulation point.
Care
should be taken with the routing of the I
S
+
and I
S
–
traces to avoid noise pickup. The traces should be Kelvin-
sensed off of the sense resistor and routed right beside
each other on an inner layer of the PCB. Avoid routing near
high voltage, high slew rate nodes such as the drain of the
primary NMOS and the drain of the active clamp PMOS.
Depending on PCB layout and the shielding of the traces
going to the I
S
+
and I
S
–
pins, it is sometimes necessary
to add a small amount of filtering as shown in Figure 5.
Typically,
values of R
FL
= 100Ω and C
FL
= 200pF to 1nF
will provide adequate filtering of noise pickup without
significantly degrading the overcurrent response time.
R
FL
R
FL
C
FL
R
SENSE
PRIMARY
NMOS
PG
I
S
+
I
S
–
LTC3765
3765 F05
Figure 5. Overcurrent Sense Filtering