lt3573 Linear Technology Corporation, lt3573 Datasheet - Page 14

• Manufacturer Part Number: lt3573
• Description: Isolated Flyback Converter Without An Opto-coupler
• Manufacturer: Linear Technology Corporation

LT3573
APPLICATIONS INFORMATION
Secondary Leakage Inductance
In addition to the previously described effects of leakage
inductance in general, leakage inductance on the second-
ary in particular exhibits an additional phenomenon. It
forms an inductive divider on the transformer secondary
that effectively reduces the size of the primary-referred
fl yback pulse used for feedback. This will increase the
output voltage target by a similar percentage. Note that
unlike leakage spike behavior, this phenomenon is load
independent. To the extent that the secondary leakage
inductance is a constant percentage of mutual inductance
(over manufacturing variations), this can be accommodated
by adjusting the R
Winding Resistance Effects
Resistance in either the primary or secondary will reduce
overall effi ciency (P
tion will be maintained independent of winding resistance
due to the boundary mode operation of the LT3573.
Bifi lar Winding
A bifi lar, or similar winding technique, is a good way to
minimize troublesome leakage inductances. However, re-
member that this will also increase primary-to-secondary
capacitance and limit the primary-to-secondary breakdown
voltage, so bifi lar winding is not always practical. The
Linear Technology applications group is available and
extremely qualifi ed to assist in the selection and/or design
of the transformer.
Setting the Current Limit Resistor
The maximum current limit can be set by placing a resistor
between the RILIM pin and ground. This provides some
fl exibility in picking standard off-the-shelf transformers that
may be rated for less current than the LT3573’s internal
power switch current limit. If the maximum current limit
is needed, use a 10k resistor. For lower current limits, the
following equation sets the approximate current limit:
14
R
ILIM
65 10 1 6
•
FB
OUT
3
( .
/R
REF
/P
A I
IN
resistor ratio.
). Good output voltage regula-
LIM
)
10
k
The Switch Current Limit vs R
formance Characteristics section depicts a more accurate
current limit.
Undervoltage Lockout (UVLO)
The SHDN/UVLO pin is connected to a resistive voltage
divider connected to V
threshold on the SHDN/UVLO pin for V
To introduce hysteresis, the LT3573 draws 2.5μA from the
SHDN/UVLO pin when the pin is below 1.22V. The hysteresis
is therefore user-adjustable and depends on the value of
R1. The UVLO threshold for V
The UVLO threshold for V
To implement external run/stop control, connect a small
NMOS to the UVLO pin, as shown in Figure 8. Turning the
NMOS on grounds the UVLO pin and prevents the LT3573
from operating, and the part will draw less than a 1μA of
quiescent current.
V
V
IN UVLO RISING
IN UVLO FALLING
(
(
Figure 8. Undervoltage Lockout (UVLO)
,
,
LT3573
SHDN/UVLO
)
GND
)
3573 F08
IN
1 22
as shown in Figure 8. The voltage
1 22
.
.
IN
V
IN
V R
falling is:
R1
R2
V R
IN
ILIM
• (
R
• (
R
2
rising is:
2
1
plot in the Typical Per-
1
R
R
2
IN
)
2
RUN/STOP
CONTROL
(OPTIONAL)
)
rising is 1.22V.
2 5
.
µA R
•
3573f
1