lt3573 Linear Technology Corporation, lt3573 Datasheet - Page 15

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

APPLICATIONS INFORMATION
Minimum Load Requirement
The LT3573 obtains output voltage information through
the transformer while the secondary winding is conducting
current. During this time, the output voltage (multiplied
times the turns ratio) is presented to the primary side of
the transformer. The LT3573 uses this refl ected signal to
regulate the output voltage. This means that the LT3573
must turn on every so often to sample the output voltage,
which delivers a small amount of energy to the output.
This sampling places a minimum load requirement on the
output of 1% to 2% of the maximum load.
BIAS Pin Considerations
For applications with an input voltage less than 15V, the
BIAS pin is typically connected directly to the V
input voltages greater than 15V, it is preferred to leave the
BIAS pin separate form the V
BIAS pin is regulated with an internal LDO to a voltage of
3V. By keeping the BIAS pin separate from the input voltage
at high input voltages, the physical size of the capacitors
can be minimized (the BIAS pin can then use a 6.3V or
10V rated capacitor).
Overdriving the BIAS Pin with a Third Winding
The LT3573 provides excellent output voltage regulation
without the need for an optocoupler, or third winding, but
for some applications with higher input voltages (>20V),
it may be desirable to add an additional winding (often
called a third winding) to improve the system effi ciency.
For proper operation of the LT3573, if a winding is used as
a supply for the BIAS pin, ensure that the BIAS pin voltage
is at least 3.15V and always less than the input voltage.
For a typical 24V
will improve the effi ciency gain 4-5%.
IN
application, overdriving the BIAS pin
IN
pin. In this condition, the
IN
pin. For
Loop Compensation
The LT3573 is compensated using an external resistor-
capacitor network on the V
range of R
matics in the Typical Applications section for other possible
values). If too large of an R
more susceptible to high frequency noise and jitter. If too
small of an R
suffer. The value choice for C
of the R
may be unstable, and if too large a C
transient performance will also suffer. Transient response
plays an important role for any DC/DC converter.
Design Example
The following example illustrates the converter design
process using LT3573.
Given the input voltage of 20V to 28V, the required output
is 5V, 1A.
1. Select the transformer turns ratio to accommodate
The output voltage is refl ected to the primary side by a
factor of turns ratio N. The switch voltage stress V
expressed as:
Or rearranged to:
On the other hand, the primary side current is multiplied by
the same factor of N. The converter output capability is:
and I
V
I
the output.
N
N
D
V
OUT MAX
IN(MIN)
SW MAX
N
N
50
(
(
V
OUT
C
(
IN
P
S
V
choice: if too small a C
N V
C
OUT
= 20V, V
= 1A
= 50k and C
(
)
C
)
V
N
value is used, the transient performance will
IN MAX
OUT
( (
(
0 8 1
V
V
V
. • (
OUT
IN
F
IN(MAX)
)
)
V
N V
F
)
(
C
V
F
= 1nF (see the numerous sche-
D
OUT
C
)
C
) •
= 28V, V
value is used, the part will be
pin. Typical values are in the
C
2
1
is somewhat the inverse
C
NI
V
F
value is used, the loop
PK
) 50
OUT
C
value is used, the
= 5V, V
V
LT3573
F
= 0.5V
15
SW
3573f
is